ماده تاریک (به انگلیسی: Dark Matter)، نوعی از ماده است که فرضیه وجود آن در اخترشناسی و کیهانشناسی ارائه شدهاست تا پدیدههایی را توضیح دهد که به نظر میرسد ناشی از وجود میزان خاصی از جرم باشند که از جرم موجود مشاهدهشده در جهان بیشتر است. مادهٔ تاریک بهطور مستقیم با استفاده از تلسکوپ قابل مشاهده نیست، مشخصاً مادهٔ تاریک نور یا سایر امواج الکترومغناطیسی را به میزان قابل توجهی جذب یا منتشر نمیکند. به بیان دیگر مادهٔ تاریک به سادگی مادهای است که واکنشی نسبت به نور نشان نمیدهد. در عوض، وجود و ویژگیهای مادهٔ تاریک را میتوان بهطور غیرمستقیم و از طریق تأثیرات گرانشیاش بر روی ماده مرئی، تابش و ساختار بزرگ مقیاس جهان نتیجه گرفت. طبق دادههای تیم مأموریت پلانک در سال ۲۰۱۳ و بر پایه مدل استاندارد کیهانشناسی، کل جرم-انرژی موجود در جهان شناختهشده شامل ۴٫۹٪ ماده معمولی، ۲۶٫۸٪ مادهٔ تاریک و ۶۸٫۳٪ انرژی تاریک تشکیل شدهاست. یعنی مادهٔ تاریک ۲۶٫۸٪ کل ماده موجود در جهان را تشکیل میدهد و انرژی تاریک و مادهٔ تاریک روی همرفته ۹۵٫۱٪ از کل محتویات جهان را تشکیل میدهند.
اختر-فیزیکدانان فرضیه مادهٔ تاریک را مطرح نمودند تا اختلاف میان جرم محاسبهشده برای اجرام غولپیکر آسمانی توسط دو روش استفاده از تأثیرات گرانشی آنها یا استفاده از مواد درخشان درون آنها (ستارگان، گاز، غبار) را توضیح دهند. این فرضیه نخستین بار توسط یان اورت در سال ۱۹۳۲ برای توضیح سرعتهای مداری ستارگان در کهکشان راه شیری و توسط فریتز زوییکی در سال ۱۹۳۳ برای توضیح شواهد مربوط به «جرم گمشده» در سرعتهای مداری کهکشانها در خوشههای کهکشانی، مطرح گردید. در پی آن بسیاری از مشاهدات دیگر نیز مطرح گشت که دلالت بر وجود مادهٔ تاریک در جهان داشتند. از جمله این مشاهدات میتوان به مشاهده سرعتهای چرخشی کهکشانها توسط ورا روبین در دهههای ۱۹۶۰–۱۹۷۰، همگرایی گرانشی اجسام پسزمینه توسط خوشههای کهکشانی همچون خوشه گلوله، الگوهای ناهمسانگردی دما در تابش زمینه کیهانی اشاره نمود. کیهانشناسان توافق نظر دارند که مادهٔ تاریک عمدتاً از نوعی ذره زیراتمی ناشناخته تشکیل شدهاست. جستجو برای یافتن این ذره با استفاده از وسایل گوناگون یکی از تلاشهای اصلی فیزیک ذرات بنیادی است.
اگرچه وجود مادهٔ تاریک بهطور عمومی توسط جامعه علمی مورد پذیرش قرار گرفتهاست، اما نظریههای جایگزینی نیز برای گرانش ارائه شدهاند. مثلاً میتوان به دینامیک نیوتونی اصلاحشده (مخفف انگلیسی: MOND) یا گرانش تانسور-بردار-اسکالر (مخفف انگلیسی: TeVeS) اشاره نمود که سعی در توضیح این مشاهدات غیرمعمولی بدون نیاز به معرفی جرم اضافی را دارند.
وجود مادهٔ تاریک از آثار گرانشی آن بر روی ماده مرئی و همگرایی گرانشی تابش پسزمینه نتیجهگیری میشود و فرضیه آن نخستین بار به این منظور مطرح شد که اختلاف میان محاسبات جرم کهکشانها و کل جهان از دو روش استفاده از دینامیک و نسبیت عام یا از طریق جرم مواد روشنی (ستارهها و گاز و غبار میانستارهای و ماده میانکهکشانی) که این اجرام دربردارند را توضیح دهد.
پذیرفتهشدهترین توضیح برای این پدیده این است که مادهٔ تاریک وجود دارد و به احتمال زیاد از ذرات سنگین با برهمکنش ضعیفی تشکیل شدهاند که تنها از طریق گرانش و نیروی هستهای ضعیف برهمکنش دارند. توضیحات جایگزین دیگری نیز پیشنهاد شدهاند که هنوز شواهد تجربی کافی برای اطمینان یافتن از اینکه کدام نظریه درست است، در دست نیست. آزمایشهای بسیاری در راه هستند برای اینکه بتوانند ذرات مادهٔ تاریک را توسط روشهای غیر گرانشی آشکارسازی کنند.
بنا بر مشاهدات ساختارهای بزرگتر از سامانههای ستارهای و همچنین مدل ریاضی کیهانشناسی مهبانگ با معادلات فریدمان و متریک فریدمان-لومتر-رابرتسون-واکر، مادهٔ تاریک ۲۶٫۸٪ کل محتوای جرم-انرژی جهان قابل مشاهده را تشکیل میدهد. در مقایسه، ماده معمولی (باریونی) تنها ۴٫۹٪ از این محتوای جرم-انرژی را تشکیل میدهد و باقی آن نیز از انرژی تاریک تشکیل شدهاست. از این ارقام چنین نتیجه میشود که در کل ماده ۳۱٫۷٪ از کل محتوای جرم-انرژی در جهان را تشکیل دادهاست و ۸۴٫۵٪ از ماده موجود نیز، مادهٔ تاریک است.
مادهٔ تاریک نقشی محوری در مدلسازی بهروز تشکیل ساختارهای کیهانی و شکلگیری و تکامل کهکشانها بازی میکند و تأثیرات قابل اندازهگیری نیز بر روی ناهمسانگردیهای مشاهدهشده در تابش زمینه کیهانی دارد. همه این ردیف شواهد حاکی از آنند که جهان به عنوان یک کل حاوی میزان مادهای بسیار فراتر از مقداری از ماده است که با امواج الکترومغناطیسی برهمکنش دارد.
مادهٔ تاریک باریونی و غیرباریونی[ویرایش]
بخش کوچکی از مادهٔ تاریک ممکن است مادهٔ تاریک باریونی باشد: اجسام نجومی مانند اجسام هالهای پرجرم فشرده (به انگلیسی: Massive Astronomical Compact Halo Objects) و (با نماد اختصاری MACHO)؛ که از ماده معمولی تشکیل شدهاند اما تابش الکترومغناطیسی آنها هیچ یا ناچیز است. با مطالعه هستهزایی در مهبانگ میتوان حد بالایی برای میزان ماده باریونی موجود درجهان تعیین نمود که نتیجه میدهد، بیشتر مادهٔ تاریک موجود در جهان نمیتواند از باریون تشکیل شده باشد و در نتیجه تشکیل اتم نمیدهد. همچنین نمیتواند از طریق نیروهای الکترومغناطیسی با ماده معمولی برهمکنشی داشته باشد. ذرات مادهٔ تاریک هیچ بار الکتریکی ندارند.
دو فرضیه در مورد ذرات مادهٔ تاریک غیر باریونی عبارتند از ذرات فرضی مانند آکسیونها یا ذرات ابرتقارنی. نوترینوها به دلیل محدودیتهای ناشی از ساختار بزرگ مقیاس و کهکشانهای با انتقال به سرخ بالا، تنها میتوانند بخش کوچکی از مادهٔ تاریک را تشکیل دهند. بر خلاف مادهٔ تاریک باریونی، مادهٔ تاریک غیرباریونی نقشی در شکلگیری عناصر شیمیایی در جهان اولیه(هستهزایی مهبانگ) نداشتهاست و به همین دلیل وجود آن تنها از طریق جاذبه گرانشیاش استنباط میگردد. علاوه بر این، اگر ذرات تشکیل دهندهاش ابرتقارنی باشند، این امکان وجود دارد که یکدیگر را نابود کنند و این نابودسازی احتمالاً به بروز عوارض قابل مشاهدهای همچون پرتو گاما و نوترینوها میانجامد(«آشکارسازی غیرمستقیم»).
مادهٔ تاریک غیرباریونی بر پایه جرم ذرات فرضی تشکیل دهندهاش یا پراکندگی سرعت این ذرات طبقهبندی میشوند. سه فرضیه برجسته در مورد مادهٔ تاریک غیر باریونی به نامهای مادهٔ تاریک سرد (CDM)، مادهٔ تاریک گرم (WDM) و مادهٔ تاریک داغ(HDM) وجود دارند. برخی از حالات ترکیبی از حالتهای فوق نیز امکانپذیر هستند. مدل مادهٔ تاریک غیرباریونی که بیش از همه مورد بحث و بررسی گسترده قرار گرفته، بر پایه فرضیه مادهٔ تاریک سرد بنا شدهاست و بنا بر پندار عمومی ذره متناظر با آن یک ذره سنگین با برهمکنش ضعیف (WIMP) است. مادهٔ تاریک داغ ممکن است شامل نوترینوهای سنگین باشد اما مشاهدات دلالت بر ان دارند که تنها کسر کوچکی از مادهٔ تاریک ممکن است داغ باشد. مادهٔ تاریک سرد منجر به شکلگیری «پایین به بالا» ی ساختار در جهان میشود، در حالیکه مادهٔ تاریک داغ منجر به تشکیل ساختار «بالا به پایین» میشود. از اواخر دهه ۱۹۹۰ مادهٔ تاریک داغ توسط مشاهدات انتقال به سرخ بالای کهکشانها مانند میدان فراژرف هابل، مردود شدهاست.
نخستین فردی که اقدام به تفسیر مشاهدات تجربی و نتیجهگیری در مورد وجود مادهٔ تاریک پرداخت، اخترشناسی هلندی به نام یان اورت بود که از پیشگامان اخترشناسی رادیویی بود و فرضیهاش را در سال ۱۹۳۲ مطرح نمود. اورت مشغول مطالعه حرکات ستارگان در منطقه کهکشانی محلی بود که دریافت که جرم در صفحه کهکشانی میبایست بیشتر از آنچه قابل دیدن است، باشد. اما بعدها مشخص گشت که این اندازهگیری اشتباه بودهاست. در سال ۱۹۳۳، فریتز زوئیکی، اخترفیزیکدان سوییسی که ضمن کار در مؤسسه فناوری کالیفرنیا، گروهها و خوشههای کهکشانی را مطالعه مینمود، نتیجهگیری مشابهی نمود. زوئیکی قضیه ویریال را در مورد خوشه کهکشانی گیسو (Coma) به کاربرد و شواهدی مبنی بر جرم گمشده بهدستآورد. زوئیکی جرم کل خوشه را بر اساس نحوه حرکت کهکشانها در نزدیکی لبههای آن تخمین زد و این تخمین را با تخمین دیگری بر پایه تعداد کهکشانها و درخشش خوشه مقایسه نمود. او متوجه شد که جرمی در حدود ۴۰۰ برابر بیشتر از آنچه دیدهمیشود وجود دارد. گرانش کهکشانهای قابل رویت در این خوشه بسیار کوچکتر از آن است که چنین مدارهای پرسرعتی بوجود آیند، بنابراین نیاز به چیزی اضافه بود. این مسئله به عنوان مسئله جرم گمشده شناخته میشود. بر پایه این نتایج زوئیکی چنین استنباط نمود که میبایست شکلی نامرئی از ماده وجود داشتهباشد که که جرم و گرانش کافی برای بهم پیوسته نگهداشتن خوشه را فراهم کند.
بیشتر شواهد مربوط به مادهٔ تاریک از مطالعه حرکت کهکشانها حاصل شدهاست. بسیاری از این حرکتها به نظر میآید که نسبتاً یکنواخت هستند، بنابراین طبق قضیه ویریال، انرژی جنبشی کل باید نصف انرژی پیوند گرانشی کهکشانها باشد. هرچندکه از نظر تجربی انرژی جنبشی مشاهدهشده بسیار بیشتر است: به بیان دقیقتر، اگر فرض کنیم که جرم گرانشی موجود تنها ناشی از ماده مرئی موجود در کهکشانهاست، ستارگانی که از مرکز کهکشان دور هستند سرعتهایی به مراتب بالاتر از آنچه قضیه ویریال پیشبینی میکند، دارند. نمودارهای منحنی چرخش کهکشانی که سرعت چرخش بر اساس فاصله را نمایش میدهند، با استفاده از ماده قابل رویت به تنهایی قابل توضیح نیستند. این پندار که ماده قابل رویت تنها بخش کوچکی از خوشه را تشکیل بدهد، سرراستترین راه توضیح این مسئله است. نشانهها بیانگر آن است که کهکشانها عمدتاً از یک هاله تقریباً کروی از مادهٔ تاریک با تمرکز بیشتر در مرکز آن تشکیل شدهاند و ماده قابل رویت مانند یک دیسک در مرکز آن قرار دارد. کهکشانهای کوتوله با درخشش سطحی کم، منابع اطلاعاتی مهمی برای مطالعه مادهٔ تاریک بهشمار میروند، زیرا در این کهکشانها نسبت ماده مرئی به مادهٔ تاریک بهطور غیرمعمولی پایین است و ستارگان پرنور کمی در مرکز آنها قراردارند که اگر چنین نبود مشاهدات منحنی چرخش ستارگان بیرونی با مشکل مواجه میشد.
مشاهدات همگرایی گرانشی خوشههای کهکشانی امکان تخمین مستقیم جرم بر پایه تأثیر آن بر نور کهکشانهای پس زمینه، فراهم میکند. تودههای عظیم ماده (تاریک یا معمولی) از طریق گرانش موجب خمش نور میشوند. در خوشههایی مانند آبل ۱۶۸۹، مشاهدات همگرایی تأیید میکنند که میزان ماده موجود به میزان قابل توجهی بیشتر از آن مقداری است که از نور این کهکشانها استنباط میشود. در خوشه گلوله، مشاهدات همگرایی بیانگر آنند که بیشتر جرمی که موجب همگرایی میشود از جرم باریونی منتشرکننده پرتو ایکس، مجزاست. در ژوئیه ۲۰۱۲ از مشاهدات همگرایی در کشف یک رشته مادهٔ تاریک بین دوخوشه کهکشانی استفاده شد که توسط شبیهسازیهای کیهانی پیشبینی شدهبود.
پیوند به بیرون[ویرایش]
Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.[note 1] Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.
The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter. Other lines of evidence include observations in gravitational lensing, from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions, and from the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 85%[note 2] of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content.
Because dark matter has not yet been observed directly, if it exists, it must barely interact with ordinary baryonic matter and radiation, except through gravity. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles (WIMPs), or gravitationally-interacting massive particles (GIMPs). Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded. Dark matter is classified as cold, warm, or hot according to its velocity (more precisely, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.
Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists, intrigued by certain observations that do not fit the dark matter theory, argue for various modifications of the standard laws of general relativity, such as modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter.
The hypothesis of dark matter has an elaborate history. In a talk given in 1884, Lord Kelvin estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the center of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars. Lord Kelvin thus concluded that "many of our stars, perhaps a great majority of them, may be dark bodies". In 1906 Henri Poincaré in "The Milky Way and Theory of Gases" used "dark matter", or "matière obscure" in French, in discussing Kelvin's work.
The first to suggest the existence of dark matter, using stellar velocities, was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.
In 1933, Swiss astrophysicist Fritz Zwicky, who studied galaxy clusters while working at the California Institute of Technology, made a similar inference. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass that he called dunkle Materie ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together. This was the first formal inference about the existence of dark matter. Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant; the same calculation today shows a smaller fraction, using greater values for luminous mass. However, Zwicky did correctly infer that the bulk of the matter was dark.[clarification needed]
Further indications that the mass-to-light ratio was not unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula (known now as the Andromeda Galaxy), which suggested that the mass-to-luminosity ratio increases radially. He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to the missing matter that he had uncovered. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50, in 1940 Jan Oort discovered and wrote about the large non-visible halo of NGC 3115.
Vera Rubin, Kent Ford and Ken Freeman's work in the 1960s and 1970s, provided further strong evidence, also using galaxy rotation curves. Rubin and Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. This result was confirmed in 1978. An influential paper presented Rubin and Ford's results in 1980. They showed that most galaxies must contain about six times as much dark as visible mass; thus, by around 1980 the apparent need for dark matter was widely recognized as a major unsolved problem in astronomy.
At the same time that Rubin and Ford were exploring optical rotation curves, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (HI) often extends to much larger galactic radii than those accessible by optical studies, extending the sampling of rotation curves—and thus of the total mass distribution—to a new dynamical regime. Early mapping of Andromeda with the 300-foot telescope at Green Bank and the 250-foot dish at Jodrell Bank already showed that the HI rotation curve did not trace the expected Keplerian decline. As more sensitive receivers became available, Morton Roberts and Robert Whitehurst were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii, Figure 16 of that paper combines the optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the HI data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic HI spectroscopy was being developed. In 1972, David Rogstad and Seth Shostak published HI rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended HI disks.
A stream of observations in the 1980s supported the presence of dark matter, including gravitational lensing of background objects by galaxy clusters, the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics.
In standard cosmology, matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3. This is in contrast to radiation, which scales as the inverse fourth power of the scale factor ρ ∝ a−4 , and a cosmological constant, which is independent of a. These scalings can be understood intuitively: for an ordinary particle in a cubical box, doubling the length of the sides of the box decreases the density (and hence energy density) by a factor of eight (23). For radiation, the decrease in energy density is larger because an increase in scale factor causes a proportional redshift. A cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.
In principle, "dark matter" means all components of the universe that are not visible but still obey ρ ∝ a−3. In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons." Context will usually indicate which meaning is intended.
Galaxy rotation curves
The arms of spiral galaxies rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the Solar System.[note 3] From Kepler's Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed. Instead, the galaxy rotation curve remains flat as distance from the center increases.
If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude that the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.
Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.
As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.
Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:
Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.
One of the consequences of general relativity is that massive objects (such as a cluster of galaxies) lying between a more distant source (such as a quasar) and an observer should act as a lens to bend the light from this source. The more massive an object, the more lensing is observed.
Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including Abell 1689. By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters. Lensing can lead to multiple copies of an image. By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster.
Weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses from vast galaxy surveys. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements. Dark matter does not bend light itself; mass (in this case the mass of the dark matter) bends spacetime. Light follows the curvature of spacetime, resulting in the lensing effect.
Cosmic microwave background
Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the CMB by its gravitational potential (mainly on large scales), and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the cosmic microwave background (CMB).
The cosmic microwave background is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The series of peaks can be predicted for any assumed set of cosmological parameters by modern computer codes such as CMBFast and CAMB, and matching theory to data, therefore, constrains cosmological parameters. The first peak mostly shows the density of baryonic matter, while the third peak relates mostly to the density of dark matter, measuring the density of matter and the density of atoms.
The CMB anisotropy was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003–12, and even more precisely by the Planck spacecraft in 2013–15. The results support the Lambda-CDM model.
The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the Lambda-CDM model, but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND).
Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure. If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.
Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.
If dark matter does not exist, then the next most likely explanation is that general relativity—the prevailing theory of gravity—is incorrect and must be modified. The Bullet Cluster, the result of a recent collision of two galaxy clusters, provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass. Standard dark matter models can easily explain this observation, but modified gravity has a much harder time, especially since the observational evidence is model-independent.
Type Ia supernova distance measurements
Type Ia supernovae can be used as standard candles to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past. The data indicates that the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy. Since observations indicate the universe is almost flat, it is expected that the total energy density of everything in the universe should sum to 1 (Ωtot ~ 1). The measured dark energy density is ΩΛ = ~0.690; the observed ordinary (baryonic) matter energy density is Ωb = ~0.0482 and the energy density of radiation is negligible. This leaves a missing Ωdm = ~0.258 that nonetheless behaves like matter (see technical definition section above)—dark matter.
Sky surveys and baryon acoustic oscillations
Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon-baryon fluid of the early universe, and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~ 1 percent) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130 or 160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey. Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe. The results support the Lambda-CDM model.
Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding but more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. The effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures assuming Earth is not at a special location in the Universe.
In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models. These constraints agree with those obtained from WMAP data.
Composition of dark matter: baryonic vs. nonbaryonic
There are various hypotheses about what dark matter could consist of, as set out in the table below.
Dark matter can refer to any substance that interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.[note 4] However, for the reasons outlined below, most scientists think the dark matter is dominated by a non-baryonic component, which is likely composed of a currently unknown fundamental particle (or similar exotic state).
Baryons (protons and neutrons) make up ordinary stars and planets. However, baryonic matter also encompasses less common non-primordial black holes, neutron stars, faint old white dwarfs and brown dwarfs, collectively known as massive compact halo objects (MACHOs), which can be hard to detect.
However, multiple lines of evidence suggest the majority of dark matter is not made of baryons:
Candidates for non-baryonic dark matter are hypothetical particles such as axions, sterile neutrinos, weakly interacting massive particles (WIMPs), gravitationally-interacting massive particles (GIMPs), supersymmetric particles, or primordial black holes. The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses—however uncertain they may be—are almost certainly tiny, they can only supply a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.
Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis) and so its presence is revealed only via its gravitational effects, or weak lensing. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection).
Dark matter aggregation and dense dark matter objects
If dark matter is composed of weakly-interacting particles, an obvious question is whether it can form objects equivalent to planets, stars, or black holes. The answer has historically been that it cannot, because of two factors:
In 2015–2017 the idea that dense dark matter was composed of primordial black holes, made a comeback following results of gravitation wave measurements which detected the merger of intermediate mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. However this was later ruled out by a survey of about a thousand supernova which detected no gravitational lensing events, although about 8 would be expected if intermediate mass primordial black holes accounted for the majority of dark matter. The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the suns heliosphere by the Voyager 1 spacecraft. Tiny black holes are theorized to emit Hawking radiation. However the detected fluxes were too low and did not have the expected energy spectrum suggesting that tiny primordial black holes are not widespread enough to account for dark matter. None-the-less research and theories proposing that dense dark matter account for dark matter continue as of 2018, including approaches to dark matter cooling, and the question remains unsettled.
Classification of dark matter: cold, warm or hot
Dark matter can be divided into cold, warm, and hot categories. These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion—this is an important distance called the free streaming length (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation. The categories are set with respect to the size of a protogalaxy (an object that later evolves into a dwarf galaxy): dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.
Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;[clarification needed] the latter is excluded by high-redshift galaxy observations.
Another approximate dividing line is that warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era (photons and neutrinos), with a photon temperature 2.7 million K. Standard physical cosmology gives the particle horizon size as 2ct (speed of light multiplied by time) in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light-years today (absent structure formation). The actual FSL is approximately 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10 million light-years, or 3 megaparsecs, today, around the size containing an average large galaxy.
The 2.7 million K photon temperature gives a typical photon energy of 250 electron-volts, thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV–TeV mass WIMPs, would become non-relativistic much earlier than one year after the Big Bang and thus have FSLs much smaller than a protogalaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot.
Cold dark matter
Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early.
The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holes and Preon stars) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions.
Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists that MACHOs cannot make up more than a small fraction of dark matter. According to A. Peter: "... the only really plausible dark-matter candidates are new particles."
The 1997 DAMA/NaI experiment and its successor DAMA/LIBRA in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results.
Many supersymmetric models offer dark matter candidates in the form of the WIMPy Lightest Supersymmetric Particle (LSP). Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.
Warm dark matter
Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ~ 300 eV to 3000 eV.
No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: a heavier, slower form of neutrino that does not interact through the weak force, unlike other neutrinos. Some modified gravity theories, such as scalar–tensor–vector gravity, require "warm" dark matter to make their equations work.
Hot dark matter
Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in 1956. Neutrinos' mass is less than 10−6 that of an electron. Neutrinos interact with normal matter only via gravity and the weak force, making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them 'weakly interacting light particles' (WILPs), as opposed to WIMPs.
The three known flavours of neutrinos are the electron, muon, and tau. Their masses are slightly different. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50 eV/c2 (less than 10−5 of the mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.
Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.
Detection of dark matter particles
If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second. Many experiments aim to test this hypothesis. Although WIMPs are popular search candidates, the Axion Dark Matter Experiment (ADMX) searches for axions. Another candidate is heavy hidden sector particles that only interact with ordinary matter via gravity.
These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.
Direct detection experiments aim to observe low-energy recoils (typically a few keVs) of nuclei induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil the nucleus will emit energy in the form of scintillation light or phonons, as they pass through sensitive detection apparatus. To do this effectively, it is crucial to maintain a low background, and so such experiments operate deep underground to reduce the interference from cosmic rays. Examples of underground laboratories with direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at Sudbury, the Gran Sasso National Laboratory, the Canfranc Underground Laboratory, the Boulby Underground Laboratory, the Deep Underground Science and Engineering Laboratory and the China Jinping Underground Laboratory.
These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100 mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: CDMS, CRESST, EDELWEISS, EURECA. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP, DarkSide, PandaX, and LUX, the Large Underground Xenon experiment. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include SIMPLE and PICASSO.
Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles. The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations have detected an annual modulation in the rate of events in their detectors, which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX and SuperCDMS.
A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center. A low-pressure time projection chamber makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun travels (approximately towards Cygnus) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.
Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the centre of our galaxy) two dark matter particles could annihilate to produce gamma rays or Standard Model particle-antiparticle pairs. Alternatively if the dark matter particle is unstable, it could decay into standard model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in our galaxy or others. A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.
A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy neutrinos. Such a signal would be strong indirect proof of WIMP dark matter. High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal. The detection by LIGO in September 2015 of gravitational waves, opens the possibility of observing dark matter in a new way, particularly if it is in the form of primordial black holes.
Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow. The Energetic Gamma Ray Experiment Telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity.
The Fermi Gamma-ray Space Telescope is searching for similar gamma rays. In April 2012, an analysis of previously available data from its Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way. WIMP annihilation was seen as the most probable explanation.
In 2013 results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays that could be due to dark matter annihilation.
Collider searches for dark matter
An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect dark matter particles produced in collisions of the LHC proton beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected. Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks. It is important to note that any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter.
Because dark matter remains to be conclusively identified, many other hypotheses have emerged aiming to explain the observational phenomena that dark matter was conceived to explain. The most common method is to modify general relativity. General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven. A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor-vector-scalar gravity (TeVeS), f(R) gravity, negative mass dark fluid, and entropic gravity. Alternative theories abound.
A problem with alternative hypotheses is that the observational evidence for dark matter comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity.
The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter.
In philosophy of science
In philosophy of science, dark matter is an example of an auxiliary hypothesis, an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark matter hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.
In popular culture
Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the hypothesized properties of dark matter in physics and cosmology.