Astronomy and Astrophysics

Dark Matter / Dark Energy
Subha Majumdar, Tata Institute for Fundamental Research (TIFR), Mumbai

We live in a strange and fascinating Universe. Once we take an inventory of everything that is visible, everything that makes atoms to ants to galaxies, we can only account for 5% of the total energy density in the Universe. The rest is "Dark", in the sense we cannot see them directly with our eyes or our telescopes. However, there are indirect but infallible signs that they exist - approx 25% of the composition of the Universe is Dark Matter (DM) and approx 70% is Dark Energy (DE). Yet, we have no clue as to what 'they' actually are.
Dark matter means that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. DM is not in the form of dark clouds of normal matter or baryons or antimatter or even huge black holes -- each would have its unique signature which we do not detect. However, they can still be 'massive compact halo objects', which are objects like brown dwarfs. The most accepted view is that DM is non-baryonic, something exotic like axions or WIMPS. This brings out the possibility of trying to identify the particle nature of DM in large particle physics experiments like the LHC or other dedicated DM searches. What is certain is that we need DM for us to exist, for structures to form at all scales from galaxy sized objects to the size of our entire Universe.
Dark Energy is even more bizarre! Matching observations with theory tells us that we need at least another component, a.k.a. DE, which makes up ~70% of the Universe. Yet, it is required to have properties unlike anything else we know. Two decades back, we were certain the the Universe has enough energy density (or mass) to stop its expansion or at least slow it down due to gravitational attraction. However, observations now compellingly show us that the expansion rate is growing with time, i.e. accelerating. For this to happen, we need 'something' that acts like 'anti-gravity'. In fact, since the volume of the Universe increases with expansion and coupled with the fact that the expansion is accelerating, means that this DE is becomes more dominating as time passes. For this to happen, more and more DE must be created out of nothing or 'vacuum'. So what is DE? Einstein theory of gravity has a provision for called the Cosmological Constant which states that "empty space" can possess its own energy. Or maybe, there is some strange kind of dynamical energy-fluid that fills space. Maybe, Einstein's theory of gravity is not correct. Whatever be the answer, it is bound to be stranger than fiction.

References

1. Dark Matter and Dark Energy - Marc Kamionkowski;
arxiv.org/abs/0706.2986

2. Dark Matter: A Primer - Katherine Garrett & Gintaras Duda
arxiv.org/abs/1006.2483

3. Frontiers of Dark Energy - Eric V. Linder
arxiv.org/abs/1009.1411

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