# Universal Space Expansion [duplicate]

I see there have been a lot of questions about the big bang and did it start from a singular point etc. And i agree with the answers given, that the universe is infinite and is expanding about every point within it, like a grid where the space in between the points is expanding or stretching, that is what my intuition tells me and i can picture it.

However I am at a loss as to how this expansion occurs. Einstein theories result in explanation that mass or matter interacts with space and causes the space surrounding the matter to bend and subsequently light bends travelling through the space surrounding the matter, and the change in space's structure manifests in what we observe as gravity where matter we observe attracts other matter toward it by this bent space.

So what my question is asking is what is fuelling or causing the expansion, that is if space requires work to expand it, or is it simply a fundamental property of space and matter that we are yet to have an explanation of, or at least one that I'am unaware of.

• This isn't a full fledged answer, but the following two links elaborate more on the relevant details.... (1) Metric expansion of space and (2) Dark Energy May 11, 2017 at 22:36
• Possible duplicate of What causes the expansion of space? May 11, 2017 at 23:35
• May 11, 2017 at 23:40
• Assuming matter and energy are, on cosmological scales, distributed uniformly and very few other assumptions, I believe it's the case that Einstein's field equations yield that (the metric of) space is either expanding or contracting or, more precisely, there is no (stable) static solution. I know this may not answer the deep question you have but perhaps it's of some value nonetheless. May 11, 2017 at 23:54

The straightforward, non-philosophical answer, is that it comes from General Relativity (GR) with certain observations and verifications.

First GR. Einstein's theory, as you call it, which is GR, is not that matter-energy interacts with space and causes SPACE to bend. Not exactly true, but true in some cases. it actually bends SPACETIME. TIME is also bent. We define the bending by the curvature it creates in SPACETIME. A simple mathematical depiction of the geometry of a SPACETIME is given by the formula for the spacetime interval in a geometry, written as

$$ds^2 = g_{\mu\nu} dx^\mu dx^\nu$$

where the indices $\mu$ and $\nu$ going from 1 to 4, and $dx^4$ is time, with the other 3 being x, y, z. That's the space and time coordinates. The $g_{\mu\nu}$ are the components of the metric tensor, with the repeated indices signifying that you must sure over all possible values. From the g's one can construct curvature measures that are indpenendent of the coordinate system. Thus, it is no different (well some) than space, and both will curve. Time curvature is reflected in time going slower or faster, and also as it's mixing with spatial coordinates to have space reference frames rotate or change over time.

SECOND: observations and solution

As @Alfred Centauri said in his comment, if space is homogeneous and isotropic (on a large scale, the same everywhere and in all directions), and using a simple model for the matter and energy, the Einstein equations have the solution called the FLRW solution. It identifies the metric terms g's introduced above. It turned out then that the observations leads almost surely (some 1% or so uncertainty) to all metric components being constant except,the metric components for x, y, and z being the square of a so called scale factor a(t). Initially, at the Big Bang, a(t=0) is very small, 0 or so, but it's velocity, the rate of change of a, very very large. That scale factor, as a function of time, fully determines the geometry (shape and size) of the universe. As a(t) became larger the universe grew. There were different epochs when it grew faster then slower then accelerated again, all due to the matter-energy in the universe and the equations of the FLRW solution. At a certain time t inflation started and ended quickly, due to the dark energy. See the evolution told better at https://en.m.wikipedia.org/wiki/Chronology_of_the_universe

SO, WHAT is space expansion? Simply that growth of the size of space by the factor a(t) in every direction, as it increased. We've measured it since the discovery of Hubble's law to hold for very large scales, by measuring redshift and observed distances. Also the cosmic microwave background, the CMB, confirmed a time of very high temperature and radiation, that we now observe at lower temperatures and frequencies everywhere and in all directions. It's also confirmed the quantum fluctuations sizes that later created galaxies and stars (see below).

SO, WHERE DID THE ENERGY OR IMPULSE CAME FROM? Initially from the so far unexplained BIg Bang. But that became not as big a factor in the expansion as what was called the inflationary epoch. It is believed that some quantum field, the INFLATON, decayed from a high energy state everywhere in space, and infused all the energy for the inflation, very quickly, and created all the initial matter-energy that later went into forming particles like electrons, quarks, gluons, photons and so on. Inflation is not fully verified but there is strong evidence of it from the smoothness of the universe except for inhomogeneities that grew out of Planck sized quantum fluctuations to later form galaxies and stars and planets, Andy other measurements.

The inflation theory is still waiting to be fully confirmed through more recent and future measurements and observations including from gamma rays, the rest of the EM spectrum, and gravitational radiation telescopes that will be deployed and see past the radiation wall where the CMB came from

SO DO WE KNOW ALL THAT? REALLY? It's, like most modern science, a developing field, with some items still to be verified. The inflation theory is one, dark energy another, and dark matter the third. But it all fits very nicely into the consensus LAMBDA CDM cosmology model. As we find out more we'll know more. But the expansion of the universe does not seem to be in question.

• In your answer for my question you mention that matter bends space time, and time is actually bent which results in a slowing down or speeding up of the passage of time. If matter directly causes the bending of space time as GR suggests and the passage of time is described as an arrow of time in one direction forward. Is it theoretically possible that a large enough or dense enough clump of matter is able to bend time enough so that the direction of the arrow of time goes through 180 degrees and points in the opposite direction and thus time is reversed. May 12, 2017 at 17:33
• No. It's not bending time in the sense that it turns 45 degrees. It just slows down or speeds up. An extreme example is near a black hole. As you get closer to the black hole horizon, from the point of view of someone far away, you age and move and your heart beeps slower and slower. A slow that that observer sees you getting real close to the horizon but never reaaching it, even years later. His time and your time run differently by theoretically an infinite factor. But it can never go backwards. It's just a smaller aero of time, not at any angle. Going back in time can't happen in relativity May 12, 2017 at 20:17
• So for an object such as a planet the inhabitants that occupy it the passage of time that an observer on another planet with different mass observes the passage of time of the inhabitants of the other planet to be slower or faster than the passage of time they experience dependent on whether there planets mass is greater or smaller than the other planet? May 12, 2017 at 20:33
• Yes. But the difference would be small. You'd have to have very large gravitational fields to make a big difference. It happens near black holes and neutron stars. Btw, we do measure a difference between our clocks and the gps satellite clocks: they go a tad bit faster than ours on the ground because gravity at their altitudes are slightly smaller than on earth. They have to adjust them to measure it right May 12, 2017 at 20:40
• So the passage of time is dependent upon the mass of the object you are near and the distance you are away from it relative to an observer closer to the mass because the gravity you experience is smaller from given by the law g=Gm1m2/r2 in classical mechanics check?. Now GR describes that passage of time is also dependent upon relative velocity of two observers is this also because the spacetime is bent differently for each observer relative to the other, or is it because of another reason? May 12, 2017 at 21:05