How do they know the universe is expanding

In summary: Yes, the cole sore on the defendant's lip would still be evidence that he committed the robbery, even if the time frame between when the cole sore was present and the time of the robbery was only a year.
  • #1
ukmicky
114
1
How do they know the universe is expanding when all the evidence pointing to that fact is millions or even billions of years old.
 
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  • #2
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
LURCH said:
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).

This stuff isn't 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...
 
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  • #4
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
I see, so really the notion that the universe is still expanding today is an assumtion?
 
  • #6
DB that's the way i see it. but an assumption doesn't sound very scientific to me.
 
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  • #7
It's not expanding, the space in between the objects that we can identify is increasing
 
  • #8
Lunatic said:
It's not expanding, the space in between the objects that we can identify is increasing

are you saying that space is infinite?
 
  • #9
Lunatic said:
It's not expanding, the space in between the objects that we can identify is increasing
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.:smile:


ps i like the software that this forum uses, it can be a bit slow at time's but still gets the thumbs up
 
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  • #10
ukmicky said:
The evidence of expansion is at best millions of years out of date and wouldn't stand up in court.

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.
 

1. How do we know that the universe is expanding?

Scientists have been able to determine that the universe is expanding through various observations and experiments. One of the main pieces of evidence is the redshift of light from distant galaxies. This redshift is caused by the Doppler effect, which is when the wavelength of light appears to stretch as an object moves away from us. The more distant a galaxy is, the greater the redshift is, indicating that the universe is expanding.

2. What is the Big Bang theory and how does it support the idea of an expanding universe?

The Big Bang theory is the prevailing scientific explanation for the origin and evolution of the universe. It states that the universe began as a singularity, a point of infinite density and temperature, and has been expanding ever since. This theory is supported by the observation of the cosmic microwave background radiation, which is leftover energy from the Big Bang. The expansion of the universe is also predicted by the equations of general relativity, which have been tested and proven to accurately describe the behavior of the universe.

3. How does the expansion of the universe affect the motion of objects within it?

The expansion of the universe does not directly affect the motion of objects within it. This is because the expansion is happening on such a large scale, while the gravitational forces between objects are much stronger on a smaller scale. In other words, the expansion of the universe is not strong enough to overcome the pull of gravity between objects. However, over time, the expansion will cause objects to move further apart from each other.

4. Can the expansion of the universe be observed in real-time?

No, the expansion of the universe happens on a timescale much larger than our own. The expansion rate is measured in terms of the Hubble constant, which is approximately 70 kilometers per second per megaparsec. This means that for every megaparsec (3.26 million light-years) of distance, the expansion rate is 70 kilometers per second. This rate is too slow for us to observe in real-time, but it can be measured through the redshift of light from distant galaxies.

5. Will the expansion of the universe continue forever?

Based on current observations and theories, it is believed that the expansion of the universe will continue forever. This is because the expansion is not caused by objects moving through space, but rather by the stretching of space itself. This type of expansion is not affected by the amount of matter or energy in the universe, so it is expected to continue indefinitely. However, this is still an area of ongoing research and our understanding may change as we gather more data about the universe.

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