Origin and fate of the universe
It’s been a long since we came to know that our planet is not the bigger picture while looking from a cosmological point of view. The universe as we call it found out to be the greatest mystery humans have ever come up with. Scientists say that the universe began 14 billion thousand years ago with a massive explosion called the big bang. But, is that real? Is there an exact beginning to the universe? Or will it be going to be the same all the time? These questions can be answered when science knows exactly how the universe began. Provided with much more of a bigger picture, we have to be ready to look over the sky and find what’s really out there. Perhaps itās just a curiosity that drives our interest in these matters. In doing so, this article tries to explain some of the basics principles of cosmology and studies the mysterious beginning of everything.
What is cosmology?
Cosmology is the study of the origin, evolution, and fate of the universe. It is mainly concerned about the extragalactic world.
The study of cosmology started a long time ago. The Greek philosophers Ptolemy and Copernicus have studied and contributed to a major part of cosmology. The geocentric theory by Ptolemy was the widely accepted one during his time. According to this theory, the earth was a stationary planet and was considered to be the center of the universe. The sun along with other planets and stars revolved around the earth in a circular path.
This was a prevailing theory until the 16th century when a famous scientist, Nicolas Copernicus introduced another theory called the heliocentric theory. According to this theory, all the planets revolve around a star called the sun in a circular path, which contradicts the geocentric theory as a whole.
After a few years, Johannes Kepler, who was a key figure of the 17th-century scientific revolution (Johannes Kepler, n.d.), introduced three laws which are known as Keplerās laws which supported the Copernican theory. According to his studies, the planets are revolving around the sun in orbits which are not perfect circles but are elliptical.
Modern cosmology
Modern cosmology began around 1917 when Albert Einstein introduced the general theory of relativity (Einstein ushers in modern cosmology, n.d.). This became a huge turning point. Einstein believed that the universe is unchanging and static and introduced a set of equations that explained gravity. But, later he considered the idea of a static and unchanging universe was a big blunder! Later, in 1922 Alexander Friedman introduced the idea of the expanding universe. This explained a lot of unsolved phenomena like cosmological redshift (shifting of wavelength to the red part of the spectrum as the galaxies move away from each other) which was detected by Edwin Hubble in 1929.
Hubbleās law
Hubbleās law states that galaxies are moving away from the Earth at velocities proportional to their distance.
\(v=\frac{d}{H}\)
Where d is the proper distance to the galaxy and H is the Hubble’s constant. The age our universe is the reciprocal of Hubble’s constant (1H). Which is also called Hubble’s Time which is 14.4 billion years.
The standard model of cosmology
Our quest for a perfect model that could explain the whole universe was started a long time back and continues. Among them, the standard model of cosmology was a model that could explain most of the observed features of our current observable universe.
Standard cosmology is based on two important theories of all time – Einstein’s general theory of relativity and the cosmological principle. Einstein’s general theory of relativity is a geometric theory of gravitation which is providing a unified description of gravity as a property of space and time, or together known as space-time. The cosmological principle on the other hand states that when viewed on a large scale, the universe looks homogeneous and isotropic.
Einstein’s general theory of relativity includes a set of partial differential equations known as field equations.
\(R_{\mu\theta}-\frac{1}{2}g_{\mu\theta}R=\frac{8\pi G}{c^4}T_{\mu\theta}\)
\(R_{\mu\theta}-\frac{1}{2}g_{\mu\theta}\) is the Einstein tensor \(G_{\mu\theta}\), which gives the geometry and is determined by the source of gravity \(T_{\mu\theta}\) is called the Energy Momentum Tensor. (Baumann, p. 16)
Friedmann-Lemaitre-Robertson metric (FRW metric) explains about the homogeneous, isotropic, expanding universe that is path-connected.
The FLRW metric is given by (with c=1),
\(ds^2= a(t)^2[\frac{dr^2}{1-Kr^2}+r^2d\Omega^2]\)
where a is the scale factor which is a dimensionless factor of relative expansion of the universe.
It is a fact that the standard model was successful in explaining a lot about our universe, but it has a lot of drawbacks in explaining various problems like horizon problem, flatness problem, singularity problem, etc.
Problems…!
Horizon’s problem raises the question of how two disconnected patches of space have very nearly the same temperature and densities as mentioned in the cosmological principle. The flatness problem has to do with the geometry of the universe, which appears to be a flat geometry. The matter density of the universe is so close to the unstable critical value between everlasting expansion and re-collapse into a big crunch. The singularity problem comes at t=0 (initial time of universe) which is an infinite energy density state, so general relativity predicts its breakdown. Since the standard cosmology is unable to explain these problems a new idea called inflation is introduced.
Inflation
Inflation is the exponential expansion of the universe at the evolution stage of the early universe. It solved mainly the horizon problem and flatness problem. The horizon problem is solved because the different disconnected patches of space we see used to be close enough to communicate, but during the inflation, spaces expanded and these close regions spread out into infinity. The flatness problem was resolved by a simple idea of a balloon, when the balloon expands, though, the surface smoothes out and becomes as flat. Similarly, inflation flattens the universe.
Geometry of universe
General theory of relativity is used to determine the curvature of the universe by using a value called the density parameter (Ī©).
if the actual density is equal to this critical density( Ī© = 1), then the universe is flat. If Ī© > 1, there is positive curvature and ,if Ī© < 1 there is negative curvature.
The expansion of the Universe during the inflationary epoch serves as quantum fluctuations in the inflation fields and these quantum fluctuations lead to the primordial perturbations. This leaves imprints in the cosmic microwave background radiation which is the light from the early universe and in the distribution of galaxies.
Big bang and Cosmic microwave background
In 1927, an astronomer named Georges Lemaitre came up with the idea of a big bang. He said that our universe started from a single point with infinite density. After the explosion, the universe began to expand exponentially and got bigger and bigger.
During the initial time, the universe was so hot, but after 20 minutes it came to a stage where the universe was no longer hot enough to occur fusion, but far too hot for neutral atoms to exist. At that time the universe was filled with opaque plasma (charged particles). At about 380,000 years later the universe became cool enough and particles combined to form neutral atoms. This period is called the recombination period. As a result of recombination, the universe became transparent and the atoms like hydrogen, helium, and lithium came to their ground state by releasing photons which are called photon decoupling. These photons can be seen even today as a cosmic microwave background.
Initially, these photons were too hot but now it reduced into a temperature of 2.72 K (Planck temperature). CMB is currently the oldest observation we have in the universe.
Dark matter and dark energy
Expansion of the universe was proved in the early 1990s by the scientists. They got valid theories that the universe had exploded somewhere in the past, and all the matter and energy was confined at a point. The energy density of the universe is at a critical point that scientists think that the universe is slowing down its expansion rate as time goes due to gravity, but in reality it is accelerating. To explain this, scientists introduced the idea of dark energy and dark matter.
If we calculate the amount of all matter present in our universe, it will only add up to 5% of the universe. So what fills the universe? It’s dark energy and dark matter. Up to 68% of the universe is filled with dark energy which is unknown energy, and its presence can be felt by the expansion rate of our universe. dark matter adds up to 27% of the universe which is the matter which cannot be seen through electromagnetic radiation because they don’t emit light.
Fate of the universe
The fate of the universe is still a mystery. Scientists came up with a lot of interesting theories. Big freeze, big rip and big crunch are some of the convincing theories among them.
The Big freeze theory says that the whole universe will expand indefinitely and at some point, the temperature of the universe will asymptotically reduce to absolute zero and the expansion will stop.
The Big crunch is another scenario in which the whole universe will accelerate backwards and eventually recollapses to the starting point of the universe.
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