Expansion of Universe
Many scientists considered that the spiral nebulae in the sky were distant galaxies before 19th century. It was Edwin Hubble’s work in the 1920’s that showed this was not only true, but also the more distant a galaxy was, the faster it is receding away from us. Ipso facto, Hubble’s Law, which described the expansion of the Universe , led to a very straightforward interpretation which is consistent with the Big Bang model. It presumed if the Universe is expanding today, then it was smaller and denser in the past.
Hubble suggested that the universe as the whole is expanding that infers to the red shifts in spectra of galaxies. It thus suggested that the observed red shift of the galaxy is directly proportional to the distance (D) of galaxy from earth.
So, Z ∝ D
or, Z = HD
Here H is proportionality constant and if galaxy is receding at the pace of velocity ‘v’ then, Z = (v ⁄ c), where ‘c’ is the velocity of light through free space.
so, v = H₀ D
H₀ is the Hubble’s constant and the expression is called Hubble’s law.
Consider T is time expensed by the universe from its creation and ‘v’ as the velocity of recessing galaxy Assuming that all the galaxies were in a small singularity at time t=0 sec from its creation, they has covered the distance r till today.
According to Hubble’s law,
v = H₀ r
or, v= H₀ (v × T)
or, T = 1 ⁄ H₀
Hence the age of universe is mathematically the reciprocal of Hubble’s constant which is 13.8 billion year.
Later Soviet Physicist Alexander Friedmann came up with the mathematical model in 1922, when this idea was criticized by Einstein. Lemaître’s 1927 work was also dismissed by Einstein, and even after Hubble’s work in 1929, the idea that the Universe was smaller, denser, and more uniform in the past was a fringe idea in scientific community. But Lemaître added the idea that the red shifts of galaxies could be explained by the expansion of space, and that there must have been an initial “moment of creation” at the beginning. It was known as either the “primeval atom” or “cosmic egg” for decades.
According to cosmologists the most likely possible fates for the Universe is Big Freeze or Big Crunch. The evolution of the universe is determined by the struggle between the momentum of expansion and the pull of gravity. The current rate of expansion is measured by the Hubble constant, while the strength of gravity depends on density and pressure of the matter in the universe. If the pressure of the matter is low, then the fate of universe is governed by the density. If the density of the universe is less than critical density, then the universe will expand forever, there isn’t enough gravitational pull to stop the outward expansion. This is known as Big Freeze, because the universe will cool as it expands, every star will die, all matter will decay into a sparse soup of particles and radiation. the heat death. If the density of the universe is greater than the critical density, the gravity will win and the universe will collapse bach on itself, the so called Big Crunch. Recent observations of distant supernova have suggested that the expansion is speeding up, this implies the existence of matter referred to as ‘dark energy’. Gravity slows down the expansion, dark energy speeds it up.
There are three possibilities :
1. Open Universe ; the universe will expand forever, the matter it contains will spread thinner, it will become dark, cold, and lifeless.
2. Flat universe
3. Closed Universe ; its expansion will slow down until it reaches a maximum size;then it will recoil, collapsing back on itself, will become infinitely hot and infinitely dense singularity
The astronomers know the universe is expanding and can’t gauge the force responsible for expansion ; instead try to measure the density of the universe. The higher the density the greater the gravitional force. Applying this idea, there must be a density threshold — a critical limit-that will determine if the gravity within the universe is strong enough to stop expansion.
Τhe parameter Ω(t), is given by the following equation :
Ω = ρ / ρₜ
Where ρ is actual density of universe and ρₜ is critical density.
If Ω > 1, The universe will stop expanding and start contracting, universe will be closed.
If Ω < 1 the universe will expand forever.
If Ω =1 the universe will be flat
But cosmologist think there is another type of matter that can’t be seen that can may enough gravity to stop expanding universe, the dark matter while dark energy works to speed up the expansion, it’s existence is predicted by Einstein. Scientists lead by mathematical physicist Roger Penrose thinks that our universe is just one of stages in the endless cycles of expansion and cooling, each of beginnings with a Big Bang, that ends with a big crunch.
Dark Energy ( reason why universe expand)
Dark Energy is the mysterious permeating force, that’s causing the rate of expansion of our universe to accelerate over time, rather than to slow down. As space expands more, more energy is needed. As a result, this would cause the universe to expand faster and faster. It is not only important for astronomy, it’s the central problem for physics. It’s the bone in our throat for a long time. On the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the Universe’s expansion accelerate. That’s contrary to what we might expect from a universe that began in a Big Bang.
Cosmological Constant
In 1917 Einstein added a cosmological constant ‘Λ’ to his field equations that acted as a cosmic repulsive force to counteract gravity for a static universe (neither expanding, nor contracting). But Einstein abandoned the idea when observations showed that the universe is expanding. For many years this remained a blunder until 1998 when Saul Perlmutter, Brian Schmidt and Adam Reis found the accelerating expansion of universe through observation of distant supernovae. In 2011 they received jointly Nobel Prize in physics for the discovery of accelerating expansion of the universe. Cosmologists suggest that dark energy responsible for these large scale structural changes of the universe is generated at the smallest scales known. Dark energy was negligible in the early universe. Dark energy is an energy density inherent to empty space or a vacuum energy. Mathematical physicist like Sir Roger penrose suggests that vacuum Energy is equivalent to Einstein’s Cosmological constant ‘Λ’. The current understanding of the vacuum energy based on quantum field theory is that vacuum energy came from quantum fluctuations ( i. e. particle antiparticle pairs that come into existence & annihilate each other shortly in empty space). The presence of dark energy causes the cosmos to expand, as it expands, new space forms and the repulsive energy increases. As space expands the dark energy density remains constant as times go on. Recent statistics from Chandra X-ray observatory shows us that the dark energy is stopping the formation of galaxies in the universe.
What about repulsive pressure of dark energy?
In, room we suffer positive pressure of air molecules about 50 pounds per square inch, which we can’t perceive because it is same everywhere. Similarly, in case of positive energy density and negative pressure of quantum vacuum state, Einstein’s fields allows us that pressure gravitates as well as the energy. Negative pressure operate in 3D and it produces gravitational repulsion more than the energy does have gravitational attraction. Thus, overall repulsion cause space time fabric to expand. Cosmological constant introduced by Einstein in his Einstein field equation is taken as energy in vacuum energy state of universe. There was accelerated expansion of universe called , Inflation. The energy was ethereal about 10⁷⁷ gram per cubic centimeter. Universe doubled by time 10⁻³⁰ second, with constant energy density. The current expansion of universe of low energy inflation. As there are galaxies receding the cosmological horizon will obscure, and energy dilutes as redshifts.
