Question regarding theory on accelerating universe

[Dari]

I was wondering how Astronomers know that the universe is accelerating when what they're looking at occurred in the past? If astronomers are looking billions of years into the past then isn't it better to state that the universe was accelerating then instead of saying it is accelerating today?

 

[DirkGently1]

Dopler shift is how we know, and it is still accelerating today. Astronomers aren't always looking 'Billions of years' into the past. If you look at the Sun, you are looking eight minutes into the past yourself. Look at our nearest star neighbour and it's four years.

 

[Biftheunderstudy]

Dopler shift, though related, is not exactly the case here.

The method which won the Nobel prize in physics this year, was by using Type 1a supernova to accurately measure distances, and thus test the Hubble's Law: v=H_0 D.

This simply states that galaxies have a recession velocity that is proportional to their proper distance (times some factor), which implies that the universe is expanding at a constant rate, or a straight line on a velocity-distance plot. Note that the recession velocity leads to something 'like' a doppler shift, but instead it should be thought of as due to the expansion of the universe and so we call it a cosmological redshift.

What they found, was that by using Type 1a supernova to measure the distance to very far away galaxies, that that line deviated in such a way as to suggest an accelerating universe. This implies a cosmological constant and dark energy and all that fun stuff.

 

[Dari]

Dopler shift is how we know, and it is still accelerating today. Astronomers aren't always looking 'Billions of years' into the past. If you look at the Sun, you are looking eight minutes into the past yourself. Look at our nearest star neighbour and it's four years.

Dopler shift, though related, is not exactly the case here.

The method which won the Nobel prize in physics this year, was by using Type 1a supernova to accurately measure distances, and thus test the Hubble's Law: v=H_0 D.

This simply states that galaxies have a recession velocity that is proportional to their proper distance (times some factor), which implies that the universe is expanding at a constant rate, or a straight line on a velocity-distance plot. Note that the recession velocity leads to something 'like' a doppler shift, but instead it should be thought of as due to the expansion of the universe and so we call it a cosmological redshift.

What they found, was that by using Type 1a supernova to measure the distance to very far away galaxies, that that line deviated in such a way as to suggest an accelerating universe. This implies a cosmological constant and dark energy and all that fun stuff.

Thanks for the responses but how does that answer my question? Whether it's by one method or another these events that astronomers are looking at occured in the distant past. Since it takes (a long) time for light to bring us that information isn't it better to say that all these things occured at this time in the past instead of saying that this is actually occuring today?

 

[KIAman]

Biftheunderstudy answered your question. You just don't understand what Hubble's Law is.

In layman's terms, it is the law that the further something is away, the faster it is moving away from us. That is the Hubble constant.

The reason time is involved is simply because at cosmological distances, we rely on observational information that is very old because it took that long for it to reach us.

We look at something close. Determine the cosmological shift.
We look at something far. Determine the cosmological shift.

The something far has a higher shift than the something close.

 

[Dari]

Biftheunderstudy answered your question. You just don't understand what Hubble's Law is.

In layman's terms, it is the law that the further something is away, the faster it is moving away from us. That is the Hubble constant.

The reason time is involved is simply because at cosmological distances, we rely on observational information that is very old because it took that long for it to reach us.

We look at something close. Determine the cosmological shift.
We look at something far. Determine the cosmological shift.

The something far has a higher shift than the something close.

I do understand but it does not change the fact that what we are looking at happened billions of years ago. We are seeing it today but it occured a long time ago.

 

[TastesLikeChicken]

I do understand but it does not change the fact that what we are looking at happened billions of years ago. We are seeing it today but it occured a long time ago.

That shouldn't make any difference. Physics is based on the theory that the physical properties of the universe were set in stone immediately after the big bang and have bene unchanged since. So a Type 1A supernova from billions of years ago would have behaved exactly as one exploding at any time throughout history since the same physics govern them all. Therefore the age of the supernova is inconsequential.

Now there is a very small possibility that science is wrong about the physical properties of the universe being unchanging over time. If that's the case they have much bigger issues than figuring out the whys of an accelerating universe or what dark matter is.

 

[Daedalus685]

I do understand but it does not change the fact that what we are looking at happened billions of years ago. We are seeing it today but it occured a long time ago.

We can predict red shift, that is how we noticed it was different than we expected (Hubble's law is not linear on large scales). We know what it "should" look like given a particular expansion and we compare these models to what we see. The models take into account that the red shift occurs throughout the entire time of the lights travel.

We can't be sure the universe didn't stop accelerating at some point in the last few million years. However, we have no data to support it, the window of time it would have to have happened in is shrinking as we get better and better measurements of the acceleration from more and more local objects, and it would require a drastic change in almost all aspects of our physical understanding to be possible in the first place.

I can't tell you that anyone is 100% certain that the universe is still accelerating, or if it changes over time. The range of possible start times for the accelerations is even ridiculously broad and so is the current error in the measurement. I also can't tell you for certain your car doesn't become a duck whenever you stop looking at it for more than 15 seconds. Granted I'm a little more certain about the car thing, but the earth shattering changes to how we think the universe works would be similar in both cases.

 

[Paul98]

I do understand but it does not change the fact that what we are looking at happened billions of years ago. We are seeing it today but it occured a long time ago.

Look at what happened to our galaxy since light left the far away galaxy billions of years ago. Also see that it happens in every direction.

 

[Dari]

That shouldn't make any difference. Physics is based on the theory that the physical properties of the universe were set in stone immediately after the big bang and have bene unchanged since. So a Type 1A supernova from billions of years ago would have behaved exactly as one exploding at any time throughout history since the same physics govern them all. Therefore the age of the supernova is inconsequential.

Now there is a very small possibility that science is wrong about the physical properties of the universe being unchanging over time. If that's the case they have much bigger issues than figuring out the whys of an accelerating universe or what dark matter is.

The age is not the prime issue. The issue for me is what may have happened since that particular event occurred. If what you say is true then the universe is accelerating at a faster rate NOW than it was right after the Big Bang.

We can predict red shift, that is how we noticed it was different than we expected (Hubble's law is not linear on large scales). We know what it "should" look like given a particular expansion and we compare these models to what we see. The models take into account that the red shift occurs throughout the entire time of the lights travel.

We can't be sure the universe didn't stop accelerating at some point in the last few million years. However, we have no data to support it, the window of time it would have to have happened in is shrinking as we get better and better measurements of the acceleration from more and more local objects, and it would require a drastic change in almost all aspects of our physical understanding to be possible in the first place.

I can't tell you that anyone is 100% certain that the universe is still accelerating, or if it changes over time. The range of possible start times for the accelerations is even ridiculously broad and so is the current error in the measurement. I also can't tell you for certain your car doesn't become a duck whenever you stop looking at it for more than 15 seconds. Granted I'm a little more certain about the car thing, but the earth shattering changes to how we think the universe works would be similar in both cases.

OK.

Look at what happened to our galaxy since light left the far away galaxy billions of years ago. Also see that it happens in every direction.

Which is what exactly?

 

[TastesLikeChicken]

The age is not the prime issue. The issue for me is what may have happened since that particular event occurred. If what you say is true then the universe is accelerating at a faster rate NOW than it was right after the Big Bang.

The findings about the accelerating cosmos have been corroborated using other methods besides the initial type 1a supernova observations so the theory has pretty much been confirmed as fact.

 

[Paul98]

You accept that the observations, that the distances and times are correct? You are just trying to understand why it means that the universe is accelerating?

 

[pw38]

The age is not the prime issue. The issue for me is what may have happened since that particular event occurred. If what you say is true then the universe is accelerating at a faster rate NOW than it was right after the Big Bang.

Well, that's kinda the point. As dark matter has more and more of an effect on the space around it the apparent speed at which galaxies are moving away from each other will increase to the point that no matter how far you look you won't be able to see the nearest galaxy. Of course that's what is assumed by many. We don't really know of course but there are some pretty strong arguments for this being the case. I think what you're wanting is an answer no one can definitely give you in which that case does it really matter?

 

[pandemonium]

I think entropy, and it's ever increasing constant, will loosely provide the explanation you seek, Dari. This will root into dark matter & dark energy and how we're on the verge of discovering their purposes and existences (or at least our rudimentary explanations that provide the amounts of energy and mass missing in current universe modeling).

 

[William Gaatjes]

The universe seems to expand. But there are a lot of conflicts between the expanding universe and the big bang theory and a lot of more observations.
The universe is also accelerating as it seems from measurements done and that should not be possible according to current theories. That is why dark matter and dark energy theories have been proposed to circumvent unsolvable issues. But there is no proof for dark energy or dark matter.

As long as we build better space telescopes and research more of the behavior in space, slowly we will find the answers. But we are going to need a space telescope the size of a super carrier such as the super carrier : USS enterprise before we will find anymore ground breaking , eye opening answers.

Because a space telescope of this size will allows us to leave the solar system and see and measure more of space then we have ever done. I should mention that we do not need to aim for interstellar space, just to stay a bit on the outside of the border of the solar system. But before we can pull of such a technological feature, there are a lot of problems that need to be solved first here on Earth. To start with "equal" mentality. Perhaps we will get there, perhaps not...

 

[Biftheunderstudy]

There is no conflict between an expanding universe and the big bang theory, that same theory was used, rather, to model the expanding universe.

An accelerating universe was, in part, predicted by Einstein with the inclusion of a cosmological constant in his theory of general relativity (which he then retracted due to his personal beliefs that the universe should be static).

No one likes the idea of dark matter or dark energy, but they are ways of trying to understand cosmology without having to know the exact details of the composition.

 

[piasabird]

So if you make a stellar observation, due to the speed of light and distance, what you are seeing already happened. The Universe could be contracting and we would not know it, because we could not see it. We have difficulty determining if some asteroid is going to pass close to the earth. So anything outside of our solar system, much less in a different Galaxy is a bit of a push to study in the current time. The items in the far reaches as such we have seen with the hubble telescope, are so far away that they might not even be there anymore.

 

[William Gaatjes]

There is no conflict between an expanding universe and the big bang theory, that same theory was used, rather, to model the expanding universe.

An accelerating universe was, in part, predicted by Einstein with the inclusion of a cosmological constant in his theory of general relativity (which he then retracted due to his personal beliefs that the universe should be static).

No one likes the idea of dark matter or dark energy, but they are ways of trying to understand cosmology without having to know the exact details of the composition.

I see that i used not the proper words, it is perhaps the case that the universe should be expanding in an accelerated rate. But it seems that the acceleration is too high. At least that is what i understand of it.

But then again, i would love to see prolonged measurements during 2 complete solar cycles of 22 years of non stop measurements. Every day a snapshot of the universe 22 years in a row to see if there is any change in the acceleration. I have a gut feeling that the acceleration will have a repetitive variation in that 22 years.

 

[Dari]

OK, since my original question has not been answered (it cannot be answered considering what happened at the edge of space billions of years ago may or may not be happening at the edge of space today, which was my point) I will ask another question. If the universe is expanding, what is it expanding into? Is space itself expanding (everything is getting absolutely, not relatively, larger) or is the known universe expanding into something?

 

[William Gaatjes]

OK, since my original question has not been answered (it cannot be answered considering what happened at the edge of space billions of years ago may or may not be happening at the edge of space today, which was my point) I will ask another question. If the universe is expanding, what is it expanding into? Is space itself expanding (everything is getting absolutely, not relatively, larger) or is the known universe expanding into something?

Knowbody knows.

 

[pw38]

OK, since my original question has not been answered (it cannot be answered considering what happened at the edge of space billions of years ago may or may not be happening at the edge of space today, which was my point) I will ask another question. If the universe is expanding, what is it expanding into? Is space itself expanding (everything is getting absolutely, not relatively, larger) or is the known universe expanding into something?

You already know that we have no idea what the answer is so unless you're just into asking unanswerable questions why do you ask? No one knows. No one can know. In fact we'll more than likely never know (in our lifetime at least). I'd personally love to be at the point in human history where we can definitely answer the two questions you asked.

 

[Dari]

You already know that we have no idea what the answer is so unless you're just into asking unanswerable questions why do you ask? No one knows. No one can know. In fact we'll more than likely never know (in our lifetime at least). I'd personally love to be at the point in human history where we can definitely answer the two questions you asked.

Well if it's something we cannot know then WTF are people winning a Nobel Prize over it? It doesn't make sense.

 

[pandemonium]

Very few people understand how space-time works, and even fewer understand where those boundaries lie. The current theory is that time and space simply cease to exist at the edge of the universe. I think of it sort of like a loop; once you reach the edge you'll simply be "turned around" without even knowing it. That's my interpretation anyways. That's about as explicative as you're going to get.

People are winning Nobel prizes over expanding our feeble comprehension of where consciousness ends and say...quantum logic begins.

 

[Biftheunderstudy]

Very few people know how science works. The premise is that NOTHING is known with 100% certainty.

The Nobel prize was given for observations of the accelerating universe as we observe it, not necessarily how it is now.

The important bits here are, the energy density needed to give rise to this acceleration appears to be constant regardless of the scale factor (or volume if you like) of the universe. There is no reason to believe that the cosmological constant should change in time, and all observations point to it being constant, thus we set it as a constant. It could change, and if there is evidence for it to change, we will adjust the model.

Now an analogy:
Suppose you measure g on one side of the Earth, is it the same on the other side of the Earth? It could change, you can't be 100% sure that its the same.
We do however have a pretty good theory for gravity, and it says that it should be the same over there, so we accept that it is and move on. This accelerating universe is kind of in the same ball park, admittedly our theory is not "pretty good" its only decent or ok.

@William Gaatjes
What's going on in or on the sun that could affect the light we receive from Supernova that are nearby compared to those that are far away? Or the baryonic acoustic oscillations in the power spectrum of the CMB? Or gravitational lensing of galaxy clusters?

There is a lot of evidence, its kind of far fetched that something in the sun is affecting such different measurements to the exact same conclusion.

 

[Dari]

Very few people know how science works. The premise is that NOTHING is known with 100% certainty.

The Nobel prize was given for observations of the accelerating universe as we observe it, not necessarily how it is now.

The important bits here are, the energy density needed to give rise to this acceleration appears to be constant regardless of the scale factor (or volume if you like) of the universe. There is no reason to believe that the cosmological constant should change in time, and all observations point to it being constant, thus we set it as a constant. It could change, and if there is evidence for it to change, we will adjust the model.

Now an analogy:
Suppose you measure g on one side of the Earth, is it the same on the other side of the Earth? It could change, you can't be 100% sure that its the same.
We do however have a pretty good theory for gravity, and it says that it should be the same over there, so we accept that it is and move on. This accelerating universe is kind of in the same ball park, admittedly our theory is not "pretty good" its only decent or ok.

@William Gaatjes
What's going on in or on the sun that could affect the light we receive from Supernova that are nearby compared to those that are far away? Or the baryonic acoustic oscillations in the power spectrum of the CMB? Or gravitational lensing of galaxy clusters?

There is a lot of evidence, its kind of far fetched that something in the sun is affecting such different measurements to the exact same conclusion.

But you're missing a fundamental part of my complaint. This cosmological constant or, in your analogy, g, is constant since we've been studying it, which is roughly 100 years. The universe has been around far longer. 100 years compared to 15 billion years is less than a nano-second.