B.A. Hons (Economics) seeks simple explanation! It seems to be almost completely accepted that the universe is expanding and that it is expanding increasingly rapidly. Would someone explain to me what is meant by this? Is it continually expanding in overall extent by ever more parsecs each day, or expanding faster even relative to its existing size? The explanations I have read for belief in an expanding universe fail to make this distinction clear. Do I make myself clear? Suppose I invest ten million dollars are four percent per annum compound interest. Each year the amount of interest earned increases. The speed at which I earn dollars continually accelerates, relative to my original investment. My capital increases exponentially. Is this the way the universe expands? Or is the faster expansion of the universe akin to the rate of interest continually edging up? (I realise I have not covered all the possibilities.)
Just after the BB, the universe/spacetime underwent a very rapid expansion process we call Inflation, for a very short time: This additional aspect to the standard BB model, explains the homogeneity, isotropy, and flatness of the universe we observe and that is evidenced in the general overall uniformity of the CMBR at 2.7K. After this brief and rapid inflation, the expansion slowed down to a more sedate pace, and a tussle between the gravity from the mass/energy density of the stuff in the Universe, and an unknown force we call DE, fighting the effects of gravity that is trying to collapse the universe/spacetime. There was a time when the expansion appeared to be decellerating. But even while this expansion was decellerating under gravity from the density of the mass/energy within, the larger and larger volume meant that the gravity from the mass/energy density was having a lessening effect, until the point is reached when the expansion will start apparently accelerating: The epoch we now find ourselves in. If that sounds confusing, then perhaps this is a more understandable description...... At this time DE is working to drive the Universe outwards, and matter/energy, [including DM] is trying to hold it together Right now the matter/energy side of this tug-of-war, is currently losing. This si why we appear to be accelerating in the expansion rate. In an earlier epoch though, the matter/energy had a stronger influence due to its higher overall density and the expansion was slowing.
Supplementary: While the previous post speaks of expansion and acceleration over large scales, over smaller scales such as our local group, cluster and even beyond, the matter/energy density is such that gravity is winning.
Space seems to have an inherent, fundamental energy level, and the more space there is through expansion, the more total energy is availablbe to drive the expansion. where there is matter concentration though, because there is more gravityon the outside of such entities, gravity is winning, even having it concentrate away into black holes. and we see only part of it.
Expansion of space and its VeV without limit is therefore the equivalent of a perpetual motion machine. Thank you for asking the question. I hadn't previously thought of it that way, but it most certainly would be prohibited by energy conservation laws alone, and furthermore, it wouldn't even pass the quantum test of borrowing a little energy for a long time, or a lot of energy for a short time.
Ignoring that the expansion appears to be accelerating and assuming the expansion is constant, the universe is expanding everywhere at about 70 (km/sec)/Mpc. A Mpc about 3.3 million light years. So 2 objects that are 1 mpc apart will have a recession velocity of 70 km/sec. 2 objects that are 10 mpc apart will have a recession velocity of approximately 140 km/sec. 2 objects that are 5000 mpc apart will have a recession velocity of 350,000 km/sec which is >c so the light from one of the objects will never reach the other object. The acceleration of the expansion means the Hubble 'constant' 70 (km/sec)/Mpc is not really constant and is increasing. In other words at some time in the future it will be 80 (km/sec)/Mpc or 1000 (km/sec)/Mpc
Zero energy theories of the universe, is a hypothesis and hardly established. It has been investigated by Hawking and various other scientists, but let's be clear what is fact and what is hypothesis.
So to the OP: at any given moment it is like compound interest, in that each little bit of empty space is expanding, including the space created by previous expansion. But the "interest rate" changes over time.
