The Unknown Universe

The Unknown Universe

   One of the key questions that needs to be answered by astrophysicists is what is really out there? Which immediately leads to another question, what is it all made up of? Without this understanding it is impossible to come to any firm conclusions about how the universe evolved. Everything which we encounter in our daily lives is made up of three fundamental particles: protons, neutrons and electrons (protons and neutrons are together referred as baryons). Until about thirty years ago, astronomers thought that the universe was composed almost entirely of this "baryonic matter", ordinary atoms. However, in the past few decades, there has been ever more evidence accumulating that suggests there is something in the universe that we cannot see, perhaps some new form of matter.

   Using the Wilkinson Microwave Anisotropy Probe (WMAP), scientists have been able to measure the fluctuations in the cosmic microwave background to a very high precision (relative density of baryonic and non-baryonic matter to an accuracy of better than a few percent of the overall density) which has helped in calculating the parameters of the big bang very accurately.

The fact that the mass of the non-baryonic matter and its interaction with the ordinary matter affects the details of the cosmic microwave background fluctuation spectrum, has helped in determining some of the properties of non-baryonic matter with itself.

   The data from WMAP shows that the universe is flat,  which implies that the mean energy density in the universe is equal to the critical density (within a 1% margin of error) which is equivalent to a mass density of 9.9 x 10^-30 g/cm3, which is equivalent to only 5.9 protons per cubic meter. Of this total density, we now know, the composition stands as follows:

 4.6% Atoms. The normal matter or the baryonic matter contributes only to 5% of the total energy density. Which means that all the galaxies and the intergalactic medium which we directly observe are but a small fraction of what is really out there.

 23% Cold Dark Matter. Dark matter is likely to be composed of one or more species of sub-atomic particles that interact very weakly with ordinary matter. Particle physicists have many plausible candidates for the dark matter, and new particle accelerator experiments are likely to bring new insight in the coming year.

 72% Dark Energy. The first observational hints of dark energy in the universe date back to the 1980's when astronomers were trying to understand how clusters of galaxies were formed. Their attempts to explain the observed distribution of galaxies were improved if dark energy was present, but the evidence was highly uncertain. In the 1990's, observations of supernova were used to trace the expansion history of the universe (over relatively recent times) and the big surprise was that the expansion appeared to be speeding up, rather than slowing down! There was some concern that the supernova data were being misinterpreted, but the result has held up to this day. In 2003, the first WMAP results came out indicating that the universe was flat (see above) and that the dark matter made up only ~23% of the density required to produce a flat universe. If 72% of the energy density in the universe is in the form of dark energy, which has a gravitationally repulsive effect, it is just the right amount to explain both the flatness of the universe and the observed accelerated expansion. Thus dark energy explains many cosmological observations at once. 

Fig1. The composition of the universe (present time)

      So far, no one has been able to prove conclusively the existence of either dark energy or dark matter. Though, recent reports based on the redshift measurement of supernova type Ia does seem to indicate the existence of dark energy (check “http://physics-depristine.blogspot.in/2012/05/incomplete-distant-type-ia-supernovae.html” for more details), the proof is far from conclusive.

     So it turns out that we know hardly about 5% of what makes the universe. Well, not really. Yes we do know with a level of certainty that baryonic matter contributes to roughly 5% of the contents of the universe, but we, for a large part of it have no idea where does that 5% lies. I shall explain that as we proceed.

    Although baryons are believed to be a minor constituent of the mass-energy budget of our universe, they have played a dominant role in astronomy because they are the only component that interacts directly and frequently with light. Indeed, much of modern astrophysics focuses on the production and destruction of heavenly bodies comprised of baryons. Out of the total baryonic matter formed right after the big bang, cosmic baryon census estimates have shown that in our present universe only ~6-10% of the matter has collapsed into luminous structures called galaxies, which means that this is the only fraction available for direct observation. The bulk of the baryons did not collapse and are present in circumgalactic regions and in the unvirialized large-scale intergalactic filaments (Mulchaey et al.1996; Fukugita et al.1998; Fukugita & Peebles 2004). Hydrodynamic simulations of structure formation predict that this missing portion from the baryon inventory is in a gravitationally shock-heated phase with temperatures in the range of T∼10⁵–10⁷ K and densities of nH∼(0.1–10)×10⁻⁵ cm⁻³ (Cen & Ostriker 1999; Dave et al.2001; Valageas et al. 2002). Since the temperature of these portion is in the range of 10⁵-10⁷ K, x-rays are by far the most suited band in the electromagnetic spectrum to probe them. Discovering this warm-hot intergalactic medium (WHIM) is one of the main science drivers for the Cosmic Origins Spectrograph (COS) on HST (Shull2009) and a lot of scientific missions has been for the same purpose. For example Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope(HST) and Far-Ultraviolet spectroscopic Explorer (FUSE) have helped in finding a significant fraction (~50%) of the baryon lying outside of galaxies in the nearby universe. The rest of the baryons are yet to be found. Which means that we are yet to find roughly half of the known matter of the universe, and we thought dark matter and dark energy are the only thing to be found.

    So, with no idea about the whereabouts of dark matter and dark energy and half of the baryonic matter, all that we really know about is 2% of the composition of the universe. Every one, while talking about the composition of the universe explains that universe is composed of roughly 5% of baryonic matter, but what almost everyone fails to mention is the fact that we are yet to locate the 50% of that 5%. We know so little about the universe!

   There is so much to be found, so much to explore, and so much unknown lies there. At times it frightens me to the core and strives me to understand it better and drives my passion.

References:

1.      Mulchaey, J. S., Mushotzky, R. F., Burstein, D., & Davis, D. S. 1996, ApJ,456, L5

2.      Fukugita, M., Hogan, C. J., & Peebles, P. J. E. 1998,ApJ,503, 518

3.      Fukugita, M., & Peebles, P. J. E. 2004,ApJ,616, 643

4.      Cen, R., & Ostriker, J. P. 1999,ApJ, 514, 1

5.      Dave, R., et al. 2001,ApJ, 552, 473

6.      Valageas, P., Schaeffer, R., & Silk, J. 2002,A&A,388, 741

7.      Shull, J. M. 2009, in AIP Conf. Ser. 1135, Future Directions in Ultraviolet Spectroscopy (Melville, NY: AIP),301

8.      Anand Narayanan, Bart P. Wakker, Blair D. Savage, Brian A. Keeney, J. Michael Shull, John T. Stocke, and Kenneth R. Sembach, ApJ, 721:960–974, 2010 October 1

For more details, please visit

1.      http://blogs.discovermagazine.com/cosmicvariance/2011/10/04/dark-energy-faq/

2.      http://arxiv.org/abs/1205.1045

3.      http://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium

Ahmad Ryan