– Swetha Srinivasan
How big is our universe? Does it end somewhere? Will it go on forever?
These questions have been asked by so many people, by so many civilizations throughout history. Answers to them have been sought out and these are some of the most confusing, yet interesting questions ever.
Let’s assume that the universe is finite. Then, where would our hand go if we stuck it out from the edge? This was contemplated by the Greeks. They also felt that an infinite universe was impossible. This left them with a paradox.
In the early 1800s, Heinrich Oblers argued that the universe cannot be infinite. Because, if we were to look up at the sky, and if it were infinite, then there must be a star in our line of sight. Though the apparent size might be tiny, the brightness of the small surface will still remain a constant. If the universe were filled with stars, then the night sky must also be bright. Since there are dark patches in the night sky, the universe must be finite. Newton, after having discovered gravity, knew that it was a universal, attractive force. If the universe were finite, it would be subject to gravity and would collapse onto itself.
According to Einstein, the universe should either be expanding or contracting. His equations demanded such a solution. However, he inserted a constant called the cosmological constant which cancelled the effects of gravity on a large scale. He made the assumption that the universe is static, which eventually became his biggest blunder.
The big discovery
In 1929, Edwin Hubble, made a critical discovery. He measured the relative distances of galaxies by measuring the brightness of Cepheid variable stars. He also measured the red shifts of these galaxies. The red shifts vs distance plot turned out to be linear. The redshifts increased linearly with distance. The possible explanation was that the universe is expanding.
The astronomers then realized that if the universe is expanding, then it must have been smaller in the past, and in the earliest time, it must have been just a small spot. This explains the Big Bang theory of the evolution of the universe (skyserver.sdss.org)
Fate of the Universe
All of this discussion calls for a small note on what the fate of the universe can be. There are three possible options.
If the density of the universe is greater than a critical density, gravity will take over, expansion stops and the universe collapses onto itself, what is popularly known as the Big Crunch. Such a universe is a closed universe.
If the density of the universe is exactly equal to the critical density, then the universe is basically flat. It does expand, but after a very long time, the expansion rate becomes zero.
If the density is lesser than a critical density, the universe keeps expanding, it is an open universe. This may result in a Big Rip.
What drives this expansion?
There is no such point called the center of the universe, from where the entities are expanding. Things are not moving away from the center but are moving away from each other. Space is expanding. We don’t see it within our solar system, or appreciably within our galaxy because the gravity over such regions is strong enough. But on a cosmological scale, things are moving away from each other, and the rate at which expansion is happening is not constant but is increasing. Things are moving away from each other faster and faster.
Most of our universe is dark energy, and it is said that this dark energy is what drives the expansion.
There have been many recent developments to decode this mysterious phenomenon. The Baryon Acoustic Oscillations programme and the Baryon Oscillation Spectroscopic Survey (BOSS) have made great progress.
Of BAOs and quasars
Baryon Acoustic Oscillations (BAO) are frozen relics left over from the pre-decoupling universe. They are ideal rulers for the 21st century cosmological distance measurements. The estimates provided are, for the ﬁrst time, ﬁrmly rooted in well-understood, linear physics. (Bruce and Renee)
The early universe was comprised of hot, dense plasma. This included baryons and electrons. Baryons are massive elementary particles made up of three quarks. Neutrons and protons are baryons (astro.ucla).
Since the plasma was so dense, the photons couldn’t traverse through space freely as they were subjected to Thomson scattering, thus the photons were essentially coupled to the existing matter.
Over time, the plasma cooled down and electrons combined with protons to form hydrogen. As photons interact lesser with neutral matter, they were now able to travel freely. The space became transparent to photons. The photons were decoupled.
Now, consider a perturbation originating in a dense region of primordial plasma before the decoupling happened. This contains dark matter, baryons and photons. The plasma is uniform except for this dense region.
High pressure drives the baryon-photon fluid outwards at over half the speed of light. The dark matter interacts only gravitationally, thus forms the center of the wave sphere region under consideration while the baryons and photons move outward together spherically due to pressure. As the decoupling happens, the photons decouple from the baryons and escape the moving sphere, streaming out quickly. The baryon sphere gets fixed at that distance, it stalls, having lost the motive pressure. With no more photon-baryon interaction, the only force present is the gravitational force of the dark matter and as a result, the baryons slowly start getting pulled towards the center. An equilibrium is established and finally, there are over-dense regions in both the outer sphere and the inner core. This outer shell is called the sound horizon. These are seen as anisotropies in the CMB radiation (Cosmic Microwave Background Radiation) and in the spatial distribution of galaxies. These fluctuations have evolved into today’s walls and voids of galaxies, meaning this baryon acoustic oscillation (BAO) scale is visible among galaxies today.
The BAO program basically involves finding a tracer of the mass density field and computing its 2-point function. The features in the 2-point function correspond to the sound horizon. By knowing the angle this distance subtends, one measures d(z). Comparing to the value at z ~ 103 allows us to constrain the evolution of the dark energy (astro.berkeley.edu)
Using the Baryon Oscillation Spectroscopic Survey (BOSS), two teams of physicists have improved on scientists’ understanding of the mysterious dark energy that drives the accelerating universe.
Quasars are utilized for this purpose. Quasars are astronomical objects of very high luminosity found in the centres of some galaxies and are powered by gas spiraling at high velocity into an extremely large black hole. The brightest quasars can outshine all of the stars in the galaxies in which they reside, which makes them visible even at distances of billions of light-years. Quasars are among the most distant and luminous objects known (Britannica).
Supermassive black holes which power radio galaxies and quasars play a prominent role in the evolution of galaxies. The quasars are surrounded by dust. Light leaving galaxies streams through that dust, revealing the imprint of the BAOs.
Using this data, astronomers have created the most accurate map yet of galaxies in the distant universe, offering a window into the past and, possibly, into dark energy.
BOSS uses a custom designed instrument called a spectrograph on the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico. The project aims to observe more than a million galaxies in six years(space.com).
An illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS
These baryon acoustic oscillations have now been measured in the distribution of galaxies.
Using the acoustic scale as a physically calibrated ruler, the angular diameter distance will be measured with a precision of 1% at redshifts z = 0.3 and z = 0.55. BOSS will also measure the distribution of quasar absorption lines and the cosmic expansion rate H(z). These measurements will provide demanding tests for theories of dark energy and the origin of cosmic acceleration. (sdss3.org)
Thus, we see that study of BAOs have paved a new path and a new avenue of exploration. They are providing a better understanding of how the universe works, its nature and behavior. Day by day, we are inching slowly but surely towards unearthing (or ‘ununiversing’ ) the truths and wonderous secrets of the universe.
· Journal of Astronomical History and Heritage, 17(3), 267–282 (2014), The discovery of quasars and its aftermath, K.I. Kellermann
· Baryon Acoustic Oscillations, Bruce A. Bassett & Renee Hlozek, Dark Energy, Ed. P. Ruiz-Lapuente, 2010