Expansion rate of the universe

**salhuneidi** · Apr7-06, 12:29 PM

I am not an expert on this matter but it got really interested in the physics of the expanding universe, I believe that the universe is expanding. My question is how could science know if this expansion rate is increasing or decreasing, and how can science ever measure such a rate.
Also I would like to know what science says about the idea of the universe contracting.

**SpaceTiger** · Apr7-06, 03:20 PM

Originally Posted by salhuneidi

I am not an expert on this matter but it got really interested in the physics of the expanding universe, I believe that the universe is expanding. My question is how could science know if this expansion rate is increasing or decreasing, and how can science ever measure such a rate.

Remember that the more distant the galaxy or quasar we're looking at, the further we're looking back in time. The recession rate of nearby galaxies give us an idea of the current expansion rate because we're not looking very far back in time (relative to the age of the universe) when we look at them. On the other hand, when I look at a galaxy with a redshift of 1 or 2, I'm looking at it when the universe was much younger and had a different expansion rate.

Also I would like to know what science says about the idea of the universe contracting.

The universe might contract in the future, but it's hard to say. The favorite model at the moment has it expanding indefinitely (at an ever increasing rate), but there are other models that fit the data which predict a coming contraction phase.

**salhuneidi** · Apr7-06, 03:33 PM

Thanks spacetiger, im also wondering that when the expansion rate increases doesnt this mean that all time is actually faster, and isnt time related somehow to the expansion rate.
I just need a little more info about how time is affected by the expansion and the rate of expansion and it would be great if you can direct me to some resources about these topics.
I am also interested in the idea of time reversal as a result of universe contraction, and if there is any information out there about this topic it would be great.

**SpaceTiger** · Apr7-06, 04:15 PM

Originally Posted by salhuneidi

Thanks spacetiger, im also wondering that when the expansion rate increases doesnt this mean that all time is actually faster, and isnt time related somehow to the expansion rate.

Cosmic time is defined by the clock of an observer at rest with respect to the Hubble flow or CMB. There is a relative time dilation between us and the distant galaxy (and it's considered when interpreting the observations), but we wouldn't experience time any differently now than 10 billion years ago.

**Chronos** · Apr11-06, 02:01 AM

General relativity is the short answer. Distant [high z] objects age more slowly than more nearby objects from our point of view. Sandard candles [i.e., supernova] is what convinced most scientists exansion started accelerating a while back. Figuring out why is a horse of a different color.

**Sheyr** · Apr12-06, 04:13 AM

Fine, but can one explain WHY is the universe EXPANDING?

**Chronos** · Apr12-06, 04:24 AM

When Einstein worked out his field equations, he realized his model of the universe was unstable. Thus, he added the 'cosmological constant' to force a steady state universe [which as all the rage in those days]. But he quickly realized this was an untenable solution. His steady state universe was literally balanced upon the point of his pencil. He cheerfully abandoned that idea when Hubble found evidence the universe was exanding. Although Hubble was never entirely satisfied with that conclusion, current evidence suggests he was correct.

**Sheyr** · Apr12-06, 07:00 AM

Originally Posted by Chronos

When Einstein worked out his field equations, he realized his model of the universe was unstable. Thus, he added the 'cosmological constant' to force a steady state universe [which as all the rage in those days]. But he quickly realized this was an untenable solution. His steady state universe was literally balanced upon the point of his pencil. He cheerfully abandoned that idea when Hubble found evidence the universe was exanding. Although Hubble was never entirely satisfied with that conclusion, current evidence suggests he was correct.

So, is it the cosmological constant the reason of expansion? If yes, what is than the cosmological constant?

Apr12-06, 07:18 PM

If I'm right, the cosmological constant was introduced to oppose the rate of expansion exactly opposite - so the universe was in this steady, non-expanding or contracting, state.

I think what you are after is the Hubble constant, which is the rate the universe is expanding at. But I believe that is just a value and I'm not sure how or why the universe is expanding at such a speed.

[Please don't be harsh if I'm wrong...I've only currently started to take up my own learning of this subject as my school doesn't tell me anything about it!]

**Chronos** · Apr13-06, 01:48 AM

Originally Posted by The)End)Of)The)World

If I'm right, the cosmological constant was introduced to oppose the rate of expansion exactly opposite - so the universe was in this steady, non-expanding or contracting, state.

I think what you are after is the Hubble constant, which is the rate the universe is expanding at. But I believe that is just a value and I'm not sure how or why the universe is expanding at such a speed.

[Please don't be harsh if I'm wrong...I've only currently started to take up my own learning of this subject as my school doesn't tell me anything about it!]

Welcome to PF, TEOTW! I think you grasp the essentials. Einstein's motivation for inserting the CC was to explain why the universe does not collapse. Like Newton, he realized gravity posed a serious problem in an infinitely old universe.

**UglyEd** · Apr17-06, 09:31 PM

Could the Universe be expanding simply because it is in a giant vacuum? When the universe was an infinitely small point. It was in some sort of space. A giant void? Infinite void? So after the BB the Universe is steadily expanding to fill the vacuum of the void. So then wouldnt it expand faster as it went because as matter pulls apart there is less gravitational pull between the matter. So there becomes less and less force slowing down the expansion?

**KingNothing** · Apr18-06, 12:57 AM

Originally Posted by UglyEd

Could the Universe be expanding simply because it is in a giant vacuum? When the universe was an infinitely small point. It was in some sort of space. A giant void? Infinite void? So after the BB the Universe is steadily expanding to fill the vacuum of the void. So then wouldnt it expand faster as it went because as matter pulls apart there is less gravitational pull between the matter. So there becomes less and less force slowing down the expansion?

Not quite. A vacuum would also be considered part of the universe. It is very ahrd to grasp logically, but essentially what happens is that it just expands, it doesn't expand into some space or into a void, it just expands.

**UglyEd** · Apr18-06, 07:38 AM

it just expands, it doesn't expand into some space or into a void, it just expands.

How do we know this? How do we know that what we consider to be the universe. Isnt just an explosion in a great void. The universe is steadily expanding and taking up more and more space (area) what space is it taking up?

**Garth** · Apr18-06, 08:20 AM

GR describes the expansion internally to the universe itself, within the fabric of space-tme. Empty space (the void?) itself expands. Objects within the universe move further apart, even if the universe is infinite in extent. We can say nothing about what happens beyond the universe, or though there are conjectures that other universes may exist.

Garth

**Tzemach** · Apr18-06, 11:13 AM

The second law of thermodynamics says that a system will move from order to disorder. In the case of the expansion of the universe if a quantity of matter is surrounded by nothing it will expand to fill the void, unless it has sufficient gravity to haul itself into a single lump. Because we don't really know the size of the universe we cannot really ascertain if it will expand forever or collapse. The consensus is that there is not enough gravity to collapse the universe, but there are enough mysteries to be solved that there may yet be something we have not discovered that might totally change this view.

**Garth** · Apr18-06, 11:54 AM

Originally Posted by Tzemach

The second law of thermodynamics says that a system will move from order to disorder. In the case of the expansion of the universe if a quantity of matter is surrounded by nothing it will expand to fill the void, unless it has sufficient gravity to haul itself into a single lump. Because we don't really know the size of the universe we cannot really ascertain if it will expand forever or collapse. The consensus is that there is not enough gravity to collapse the universe, but there are enough mysteries to be solved that there may yet be something we have not discovered that might totally change this view.

Welcome to these Forums Tzemach!

The second law certainly applies to a closed system, whether the whole universe constitutes such a system or not is debatable.

The expansion of the universe may itself be increasing the total number of states available and thus explain where order has come from.

The standard understanding is that the universe of matter is not increasing in a vacuum, but that rather the vacuum in which the matter is situated is increasing in volume.

The latest results from WMAP3 suggest that, if anything, there is enough total energy-density in the universe for its gravitation to 'close' the universe. However, this requires 23% to be unknown Dark Matter and another 73% to be even more unknown Dark Energy.

Whether the universe will recollapse or not depends not only on the density but also on how that DE behaves.

Garth