# I think the universe is slowing down not speeding up

1. Dec 3, 2011

### Vincent Neave

Hi,

I've never posted on a forum before but I've sent this question to various scientists and not had any replies so I thought someone here might be able to help. It may be that to a trained scientist it is obvious, but if it is could someone please explain where I have gone wrong.

The established wisdom is that the further away a galaxy is, the faster it is moving . As I understand it scientists have proposed that dark energy is probably responsible for this acceleration. However, after reading an article on dark energy, I suddenly realised that by my logic the universe isn't speeding up at all, it is actually slowing down!

My reasoning runs thus:-
(These velocities are only guesses to illustrate the reasoning)

The furthest galaxy detectable with modern instruments is about 13.1 billion light years away, just for example let's say that its velocity is point five the speed of light (v = .5c).

This is moving faster than galaxies only 1 billion light years away (v = .2c)

These, in their turn, are going faster than galaxies only 1000 light years away ( v = .005c)

If, instead of thinking a light year as away we think of it as ago, then a different sequence becomes apparent

13.1 million years ago the velocity was (v = .5C)

1 billion years ago it had slowed to ( v = .2C)

1000 years ago (v = .005C)

If everything was moving much slower a thousand years ago than it was 13.1 billion years ago, how can it be speeding up?

I know I'm wrong, I must be, I'm not a scientist I'm an artist! But for the life of me I can't work out why and it's making my brain hurt.

I think the problem is that thinking of lightyears as a measure of distance, to the layman at least, can give the impression that everything we see is part of the universe as it is now. Whereas much (if not most) of it may no longer exist in the form that we observe.

2. Dec 3, 2011

### Janus

Staff Emeritus
The fact that the further a galaxy is away the greater its measured velocity, in of itself, has no bearing on on whether or not the universe's expansion is slowing, speeding up or remaining constant. This is something that we would measure in all three cases as long as the universe was expanding. In other words, this observation alone only tells us that the universe is expanding.

To tell whether the rate of this expansion has slowed, sped up or stayed the same over time we have to look at the ratio of distance to recession rate. If it were a constant ratio for all galaxies (double the distance, double the recession), then this would indicate that the expansion rate has remained constant over time.

If the distance-recession rate ratio changes as we compare nearer galaxies to further, then this is an indication that the rate has not remained constant over time. If the ratio varies in one direction, it means the rate has slowed over time and in the other it means it has sped up.

The increasing red-shift with distance has been known about since the first third of the 20th century. The observations that led to the conclusion that the expansion rate was increasing with time wasn't made until the 1990's. The scientists that made this discovery were fully expecting to find that the rate had slowed over time due the the mutual gravitational attraction of the universe. They were trying to determine whether or not the rate of slowing was enough to ever cause the universe to stop expanding and collapse back in on itself at some future time. The fact that they discovered the opposite came as a surprise.

3. Dec 3, 2011

The flaw in your logic (and the reason your brain is hurting) is because you are assuming that the furthest galaxies, when they were nearer to the galaxies that are nearer to you now were moving at the same speed as the galaxies that are nearer to you now, and they were not, they have always moved faster.

To see this more clearly, imagine you are loosely holding a broken rubber band with knots tied at increments of 1 inch. Imagine that you are standing on the first knot. Pull the rubber band nearest the last knot at a constant velocity (.5c) and you will see that the last knot moves at .5c, the middle knot moves at .25c and the knot next to you is moving away from you slower still.

This is a simplified answer as there are other issues involved such as the expansion of the universe, the acceleration of the universe, and relitivistic effects, but in a nutshell, this is the answer to your dilemma. Hope your head feels better now.

4. Dec 4, 2011

### Vincent Neave

Thanks everyone, I shall have to read your replies several times and have a long think.

I still feel that it may be quite misleading to work out what the universe is like now, when we can only see what is happening at the present time in such a small proportion of it and beyond that, everything we see happening actually happened in the ever more distant past. I remember my astonishment, whilst reading Patrick Moore's "Boys Book of Space" in the nineteen fifties, to find that if the sun disappeared we would be unaware that it had happened for eight minutes! How much more so when we are talking of light years in billions.

But I do still think distant objects might be better decribed as billions of light years ago rather than away. Who knows where they are Now?

5. Dec 4, 2011

### Drakkith

Staff Emeritus
"Now" is specifically defined within SR and other related theories and it turns out that events can occur in different orders depending on your frame of reference.
http://en.wikipedia.org/wiki/Relativity_of_simultaneity

A: A light year is specifically defined as a unit of distance, not time. It makes no sense to change it just because you don't like it much.
B: Where they are "now" is mostly irrelevent since we can only receive information from them at light speed. If we need to know where they will be in a given amount of time we can calculate it.

6. Dec 4, 2011

### Vincent Neave

Hi,

Thanks for your replies, I really appreciate your input. I've given it more thought but feel that we are at cross purposes.

"The flaw in your logic (and the reason your brain is hurting) is because you are assuming that the furthest galaxies, when they were nearer to the galaxies that are nearer to you now were moving at the same speed as the galaxies that are nearer to you now, and they were not, they have always moved faster."

I obviously didn't explain myself clearly enough here as I was thinking exactly the opposite. My reasoning was based on the assumption that those distant galaxies were, as you say, always moving faster, therefore the further back in time one looks the greater the velocity at that time.
I have since read some more threads that call into question the velocities. So maybe it's not as absolute as I had assumed.

"To see this more clearly, imagine you are loosely holding a broken rubber band with knots tied at increments of 1 inch. Imagine that you are standing on the first knot. Pull the rubber band nearest the last knot at a constant velocity (.5c) and you will see that the last knot moves at .5c, the middle knot moves at .25c and the knot next to you is moving away from you slower still."

The problem I have with this analogy is that all the knots on the rubber band exist at the same time whereas when we look at distant objects we are looking into the far distant past.

The following analogy may seem odd but I think maybe it demonstrates the principle more clearly.

Imagine that somehow the ice cores being sampled in Antarctica enabled scientists to calculate how fast penguins could run at different points in history.

The sample from 50,000 years ago shows that penguins could run at a maximum of 50mph
The sample from 10,000 years ago shows that penguins could run at a maximum of 20mph
The sample from 1,000 years ago shows that penguins could run at a maximum of 10mph

Surely it would be logical to assume the the maximum speed of penguins was getting slower as time went on.

I think this latter analogy might give you more of an insight as to why my brain hurts!

Thanks again

7. Dec 4, 2011

### Chronos

Strictly speaking, a light year is a unit of time, not distance. When astronomers say a galaxy is 5 billion light years away from us, they are referring to its light travel time distance. In other words, it is as it appeared when the universe was about 9 billion years old. Due to the expansion of the universe, that galaxy is not and was not actually 5 billion light years in terms of proper distance [measured with rulers]. It was much closer when the light we now observe was emitted and is currently much more distant. Two other concepts are used to differentiate between these proper distances. The angular diameter distance tells us the proper distance when the light was emitted and the comoving distance expresses its estimated proper distance 'now'. For distances up to about 2 billion light years, all three distance measures yield about the same value. The light travel time distance is simply the most popular because it is the easiest to understand [albeit deceptive]. The angular diameter and light travel time distances are based on physical measurements, whereas the comoving distance is strictly a calculated distance. For a brief discussion see http://www.atlasoftheuniverse.com/redshift.html.

8. Dec 4, 2011

### Chronos

All galaxies are assumed to have roughly the same amount of proper motion, which is neglibly small compared to the speed of light. The issue centers around what is happening to the empty space between galaxies. This is what is expanding and the effect is cumulative.

9. Dec 4, 2011

### skeptic2

Let's forget the analogies for a moment and look at what really happened. When Hubble discovered that the farther a galaxy was, the faster it was receding astronomers assumed the rate of recession was approximately constant with distance so they used red shift (rate of recession) to calculate distance.

Subsequently a type of supernova was discovered (type Ia supernova http://en.wikipedia.org/wiki/Type_Ia_supernovae) that always explodes with the same brightness. Astronomers were then able to compare the brightness of the supernova to its redshift. Distant supernovae were dimmer than they should be resulting in the conclusion that they were farther than they should be as predicted by Hubble's constant. This would result if the expansion of the universe were accelerating.

10. Dec 4, 2011

### Vincent Neave

Wow,

Now that really does explain the paradox in a way that I can understand (I think)
and anyone who quotes Douglas Adams is a friend of mine!

Thanks a lot Chronos

11. Dec 7, 2011

### Ken G

In case it is still not clear to you, your analogy is flawed because it assumes all penguins of a given era move at the same speed, but this is not thought to be the case for our universe. Instead, the model is like a race in which all the runners start at the same time, and the fastest ones get farther from the starting line in any given time. Imagine you are at the starting line and are blind, but you have good hearing, so you can hear the shouts of the runners. The shouts that sound most distant to you are always those of the fastest runners-- and since sound has a finite speed, you could use your same argument to conclude that all the runners are slowing down. But that's not the case-- all the runners maintain their speed, there are just fast runners and slow runners, and the fast ones always sound farther away. All you can conclude is that the "runners are expanding", as said above-- to get more details about their speed requires some modeling assumptions and a dynamical theory to make sense of it (like general relativity).

So in other words, the model you are describing is perfectly possible-- the observations could be made to fit such a model, but you'd need to make the runners start the race at later and later times, and all the runners run at the same speed but that speed is falling with time. That model simply doesn't fit with any theory of physics that we have tested, whereas the commonly used model does.

12. Dec 14, 2011