Is the expansion of the universe same as any other increasing phenomenon? Is it really like balloon expanding? And if it is expanding, what is it expanding into?
Let’s dive back into history. Einstein gave the field equation that proposes that space and time being one unified entity, its fabric can be changed due to the presence of matter around it. When Russian physicist Alexander Freedman solved the equation, he found out that the universe of general relativity will have two types of solutions, one expanding and one contracting, depending upon the density of total material present in the universe.
But Einstein on other hand, was struggling with other problem related to same phenomenon. He also believed in the static nature of universe despite formulating Relativity that is the very antithesis of classical mechanics. But if universe is static spherical ball of vastness and everything is being pulled towards with force of gravity, wouldn’t there be some point in future that all matter fall eventually to the centre of universe and universe will finally collapse.
He was not willing to accept it, so he introduced a new mathematical constant in the equation, the cosmological constant. This constant denotes some kind of negative Gravity that will push the incoming matter pulled down by positive gravity and thus stabilizing the universe. Space is apparently filled with energy of quantum fields and this energy has a uniform negative pressure. This negative pressure is responsible for this hypothetical anti-gravity.
But in Freedman’s solution there was no need of any constant and universe was already expanding. Einstein later dropped this insertion and called it his “biggest blunder” after Edwin Hubble observed the expansion of universe astronomically and derived a mathematical relation for the same.
lan Guth’s Theory Of Inflation
Fast forward to 1979, physicist Alan Guth proposed his most famous theory of Inflation. He was actually focussing on basic questions related to cosmos such as why the universe is so vast. Another question he was worried about was if the universe started with big bang and is expanding ever since, no part of universe had ever been connected to each other yet every part has same equilibrium temperature of 2.7K i.e. the temperature of Cosmic Microwave Background (CMB) radiation around the universe.
His study showed that the big bang was followed by a period of rapid expansion in which the universe inflated exponentially and this smears out the irregularities, forming a homogenous and vast cosmos.
The volume of universe expanded by a factor of 10^78, thus explaining the vastness, and this inflationary phase was not instantly at the point of origin but 10^-32 seconds after the big bang.
There was a limited time period before this inflation started for every point in universe to have certain time of connectedness and were thermally at equilibrium. As it expanded during inflationary phases, the equilibrium remained and hence CMB is at uniform temperature all over the universe.
The Hypothetical Inflaton Field
For inflation to happen, he proposed a new hypothetical scalar field called “Inflaton field”. This hypothetical field has a false vacuum state with a large amount of vacuum energy and this creates a negative pressure similar to the cosmological constant of Einstein that will push the universe outward.
Every vacuum state corresponds to lowest possible energy present in that particular point or region but as we know there is nothing called certainty at quantum scale, so there always exist certain possibility of having some non-zero states in some vacuum states and those regions which have non-zero potential energy are called false vacuum state.
Since vacuum always tends to zero, to minimise the energy present in that region, space-time will expand. More there be the volume, less will be the density. At the beginning, there was only inflaton field and as the universe expanded, this field decayed to produce the fundamental particles we know today.
The Higgs Field and Higgs Field Value
If you have heard about Higgs field, they have similar properties of being eternally present and at false vacuum state. The particles interacting with the field gets Higgs field value which can be interpreted as acquiring mass.
Consequently, there are various proposals of treating Higgs field and hypothetical Inflaton field as one and the same field that originated at the big bang, then is expanding the universe, further decaying to form standard particles and giving mass to them. Higgs was just a hypothetical particle since 1964 until in 2012 it was physically discovered.
Accelerated Expansion of the Universe
In 1998, astrophysicist Saul Perlmutter along with his colleagues provided evidence that universe is not only expanding but expanding with an acceleration. But what is causing this acceleration? Is there any extra energy present in the universe that is pushing universe away?
The solution proposed that there is some hidden energy and it was named Dark Energy. Interesting thing is that, the same cosmological constant Einstein first added and then removed, can now finally be used. Although universe is expanding without the use of cosmological constant, this can be used to explain accelerated push.
So was Einstein right? Yes but not in the way he thought of using it. Assuming that standard cosmological model is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable Universe. But the question remains, if universe is expanding, what is it expanding into? There is nothing in the universe that is expanding, neither you, nor me and not even any heavenly bodies but it is the space-time framework of the universe that is changing.
Taking the balloon as an example, we are not outside the universe to observe it expanding, we are inside it. This posits new problems like if universe is expanding unto itself, then universe must be infinite and the answer for this is that we don’t know as of now.
The String Theory Proposal
Some theories do give a solution like string theory which presents a model of cosmology containing multiple bubble universes in the hyperspace, so each universe can expand into the extra dimension available just like cookies are baked in a microwave.
This may be true but we don’t have evidences to prove that. In science, lack of evidence doesn’t mean the absence of evidence. Let’s hope in some time in future we get the answers until then we can keep wondering.