How do they know the universe is expanding

1. Jan 10, 2006

ukmicky

How do they know the universe is expanding when all the evidence pointing to that fact is millions or even billions of years old.

2. Jan 10, 2006

LURCH

The best answer is to switch your statement from "millions or billions", to "millions and billions". Truth is, all the evidence we have shows the same story; redshift. We see redshift in objects billions of lightyears away, and in objects just a few million lightyears away. So, if our interprtation of the data (that redshift means expansion) is correct, then we can see that the universe was expanding several billion years ago, that it was still expanding a couple hundred million years ago, and yet still expanding 10 0r 20 million years ago. From this, we can conclude with considerable certainty that this condition still persists (simlpy by assuming that what the whole universe has done for the past several billion years, it did not cease to do just before we looked).

3. Jan 10, 2006

DB

This stuff isnt fresh on my mind, but wouldn't the fact that we see continuous redshifting in galaxies billions of light years away relative to galaxies millions of light years away from us imply that the universe is still expanding in our present time?

Sure the redshift we read on galaxie X is billions of years old, but the fact that galaxie Y would read a redshift of galaxie X aswell, would seem to me to imply the everything is still expanding....

Last edited: Jan 10, 2006
4. Jan 10, 2006

LURCH

I think the difficulty that would raise for someone asking this particular question is that we cannot really "observe" that galaxie Y would read a redshift today, only that it would have read one millions of years ago when it was where we see it today. But it is around this point that time-delay of observation starts to cloud my mind.

5. Jan 11, 2006

DB

I see, so really the notion that the universe is still expanding today is an assumtion?

6. Jan 11, 2006

ukmicky

DB that's the way i see it. but an assumption doesn't sound very scientific to me.

Last edited: Jan 11, 2006
7. Jan 11, 2006

Lunatic

It's not expanding, the space in between the objects that we can identify is increasing

8. Jan 11, 2006

DB

are you saying that space is infinite?

9. Jan 11, 2006

ukmicky

That's not really the issue though, the point i was trying to to raise is, if the evidence we are relying on is millions AND billions of years out of date how can it be used as evidence for what could be happening to the universe today. a lot of things can happen in a few million years much more in a billion.

if the universe in last 1 to 10 million years had stop expanding or stretching , and the process had begun in the far flung regions of the universe we wouldn't be close to viewing its effects yet.as the light /evidence of it would be billions of light years away.
The evidence of expansion is at best millions of years out of date and wouldn't stand up in court. so why is the present day expansion of the universe taken as fact when in reality we have know idea what going on.

ps i like the software that this forum uses, it can be a bit slow at time's but still gets the thumbs up

Last edited: Jan 11, 2006
10. Jan 12, 2006

SpaceTiger

Staff Emeritus
Evidence millions of years out of date wouldn't stand up in court if the timescales on which the system changed were much less than a million years. For most of the things we see in our everyday lives, this is definitely the case.

Suppose I present to a judge that, ten years ago, the defendant had a cole sore on his lip. Then, last year, a witness saw a person with a cole sore robbing a local Wal-Mart. Does my contention that the defendant had a cole sore ten years ago count as reasonable evidence against him? Of course not, we know that cole sores appear and disappear on scales much shorter than that. However, if I can generate a witness who says that the defendant had a cole sore just hours before the robbery, then it would be reasonable to think that he had one during the time of the robbery, no?

Back to the universe. To address your concern, you have to understand the energetic and dynamical scales of the problem. We can fairly reliably say that the universe has been expanding for something like ten billion years and we have models that describe how we think it has done so. Now, although we can't say this for sure within the past million or so years, we can say that the physical changes in the universe that are required to halt the expansion within the past million years are so extreme that they're rendered utterly implausible from the physical point of view. To my knowledge, there are no mainstream theories that would suggest such a serious change. Even further, the fact that we observed expansion for about ten billion years prior would make it awfully strange if the universe suddenly stopped expanding within the very recent past. That would put humans in a very special place in time, a sort of hubris that has proven to be wrong in nearly every circumstance that it has been previously suggested.

Just as cole sores don't disappear within hours, the universe doesn't change from apparent accelerated expansion to contraction (or stationarity) within a million years. It's just not plausible.