Is the Universe Expanding or Has it Already Expanded?

[C. Bernard]

Should not we say "the universe WAS expanding" rather than "IS expanding" since
the red shift augments as we go back in time to the farthest and therefore the oldest
galaxies?

 

[Drakkith]

What do you mean? To our knowledge the universe is still expanding.

 

[C. Bernard]

What i mean is:is the speed of the expansion slowing or accelerating?
It seems to me that it is slowing since it is greater as we go back in time.

 

[AlchemistK]

The expansion is increasing, and that too at an exponential rate. In fact, even faster than the speed of light.

 

[dpa]

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

 

[Drakkith]

What i mean is:is the speed of the expansion slowing or accelerating?
It seems to me that it is slowing since it is greater as we go back in time.

The recession velocity of galaxies increases as we look further back in time. The acceleration of the expansion turns out to be increasing.

The expansion is increasing, and that too at an exponential rate. In fact, even faster than the speed of light.

The rate of expansion causes objects to accelerate in velocity away from us. Since the speed of light is a velocity, not a measure of acceleration, nothing can accelerate at the speed of light.

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

In the absence of something stopping the expansion I don't see how you can say that.

 

[C. Bernard]

The speed of the expansion could be greater than the speed of light since it's the universe that is expanding, nevertheless we are measuring it with galaxies that no longer exist!
So i am still of the opinion that we should speak of it as being greater in the past.

 

[alexg]

So i am still of the opinion that we should speak of it as being greater in the past

But observation show us that it wasn't.

 

[marcus]

The speed of the expansion could be greater than the speed of light since it's the universe that is expanding, nevertheless we are measuring it with galaxies that no longer exist!
So i am still of the opinion that we should speak of it as being greater in the past.

My take on it is that we "measure the speed of expansion" by fitting a mathematical model to the data. We compare what the model says we ought to see with what we actually do see, and adjust the parameters (tweak a few knobs) to get the simplest best fit.

Are you comfortable with calculus? The model is a couple of simple equations and it generates curves---like there is one called a(t) the "scale factor". It is a number that increases with time.

the time derivative da/dt of a(t) could be written a'(t). Are you used to that prime-for-slope notation?
That is the closest thing I can think of that corresponds to the idea of "speed of expansion".

It is not a speed that you could write down in meters per second. Or write in terms of lightyears per century or whatever.

a(t) is a curve, at each time t it is a definite number that is currently around 1, and it's currently increasing by about 1/140 of one percent in a million years.
Right now today it is 1.00000
and in exactly one million years from now it will be 1.00007.

Right now today the time derivative of the scale factor is a'(t) = 0.00007 per million years. (a kind of "James Bond" number, if you like.)
When people say "expansion is accelerating" they mean that a'(t) is increasing. They do not mean that some uniquely defined SPEED is increasing. A speed is something you can express in meters per second.
You could call a'(t) a *rate* I guess, and say the *rate* is increasing.

At some times in the past we are confident that a'(t) has been extremely much bigger than the "James Bond" size it has today. And we are certainly confident that in the relatively recent past it has been LESS than today's value.
So the expansion rate---correctly expressed as a'(t) the timederivative of the scalefactor--has in the past been both bigger and smaller than it is today.

We can be pretty confident in the model (nothing in science is completely sure but this is unusually well supported) because it agrees well with masses and masses of data, millions of datapoints with more coming in all the time. And because the model is a straight shot derivation from the Einstein 1915 law of gravity, an equation which has been checked to exquisite precision by numerous experiments in the solar system.
So we don't look out and measure some particular speed which is "the speed of expansion of the universe". there is no such speed. We fit a model to a huge amount of data, we get a snug fit, and we calculate a curve a(t) and the slope of that curve is a'(t). It is not a speed but it is what popularizers and journalists call "the speed of expansion".

And that bad translation of a math quantity into words is responsible (along with other bad verbalizations) for much of the confusion.

 

[C. Bernard]

Thanks Markus for your effort and trouble in trying to make me understand that it is the rate of expansion and not the speed. Unfortunately i am very unconfortable with calculus.
Furthermore i was under the impression that the theory was a question of cosmological reshift and the stretching of wavelength discovered by Hubble.
Since it seems to be based on thousands of measures of something else i confess my ignorance and can only hope that not everybody aggrees with your explanation.

 

[marcus]

...
Since it seems to be based on thousands of measures of something else

Getting the simplest best fit model is based on many (hundreds of thousands) measurements of redshift. Once you have a model that predicts in broad outline what has been observed then that model tells you things like "speed of expansion" --- it gives you the curve a(t) which is the expansion history of the universe.

What one directly measures is redshift (and some other things like angular size, luminosity, correlation with microwave sky temperature, periodic behavior but for simplicity let's just focus on redshift). And one makes COUNTS of how many galaxies one sees in a particular ranges of redshift. How the count varies with redshift. So you make direct measurements and you get a kind of "census data". The model has to predict that. In broad outline it has to reproduce nature.

And the model also has to derive from the law of gravity that is the most accurate we known so far (1915 Gen Rel).

So then, at the end of all that---the best fit model gives you a curve. Actually because of uncertainty it gives a range of very similar curves. that curve a(t) is the expansion history. It is a picture of how the scalefactor has increased over time. The slope of that curve, at any given time, is the "speed of expansion" at that time. It started off very steep and then leveled off slightly and is now increasing gradually. It has always been positive slope---a(t) has always been climbing---but the slope has varied.

The point I'm trying to make is that we measure redshifts directly. We do not measure the slope of the scalefactor curve a(t) directly. We fit a model that matches and summarizes all that redshift census data (and other direct observation data) and then that model gives us the expansion history curve.

"Speed of expansion" has no other meaning.

 

[Khashishi]

What we are measuring is the amount of expansion that occurred between when the light was emitted and when the light finally reaches us. We assume that the redshift occurs because the light is expanded mid-flight. So yeah, this is measuring the expansion that occurred in the past, but not just at the point of emission.

 

[marcus]

What we are measuring is the amount of expansion that occurred between when the light was emitted and when the light finally reaches us. We assume that the redshift occurs because the light is expanded mid-flight. So yeah, this is measuring the expansion that occurred in the past, but not just at the point of emission.

Exactly right! When you measure a redshift number z, for some object, then
1+z is the ratio of scalefactor now to scalefactor back then when the light was emitted
1+z = a(now)/a(then)

That is one small bit of information about the curve a(t) the expansion history of the universe, and from many many such measurements one reconstructs the whole a(t) curve.

The "speed of expansion" has no other meaning besides the slope of that curve (which we do not measure directly but are able to construct by a kind of curve-fitting, I suppose you could call it, more exactly I'd call it model-fitting, to the data.)

The present slope of the the a(t) expansion history curve is
0.00007 per million years.
that is the fractional increase of any distance between two wide-separated stationary observers that occurs over the course of a million years.

 

[C. Bernard]

It seems i am beginning to see the light (no pun intended). So can we say we measure the expansion that occurred during the time it took the light to get to us?
And that it is an integral of all the various expansions that took place (which could have varied one way or the other)? And the overriding trend is an augmentation of the rate of expansion?

 

[marcus]

It seems i am beginning to see the light (no pun intended). So can we say we measure the expansion that occurred during the time it took the light to get to us?
And that it is an integral of all the various expansions that took place (which could have varied one way or the other)? And the overriding trend is an augmentation of the rate of expansion?

That sounds good to me. You could try it on other folk and see if works for them too.
Translating from a partly mathematical scheme into a purely verbal english language description is often awkward. It's likely to be either inconveniently wordy or else imperfect in some other way. But IMHO you got it right.

 

[eltodesukane]

i think i got C.bernard's argument.
The red shifts we observe are of light from stars far distant and hence are older than present. So should we not say cosmos was expanding?

It was, and it is. And it's accelerating.

 

[bill alsept]

My take on it is that we "measure the speed of expansion" by fitting a mathematical model to the data. We compare what the model says we ought to see with what we actually do see, and adjust the parameters (tweak a few knobs) to get the simplest best fit.

I think what a lot of people are saying and it seems more every day is that what we are actually seeing is not what’s happening but is being interpreted that way. The greatest scientist in the world use to look up at the heavens and knew for sure that Earth was the center of it all because it was so obvious.

 

[Chronos]

Science is about model building. It is imperfect, but, the most productive approach to date. Just because certain observations are inconsistent with a model does not mean the model is flawed, perhaps incomplete, but, not necessarily flawed. It's not like there is no such thing as inaccurate deductions drawn from observation, or simple observational errors. If you put your pants on backwards it is tempting to blame the tailor for the poor fit.

 

[John15]

I have the same problem with redshift. I have seen the graphs and yes speed over distance shows acceleration but change distance for time and you get a deceleration i.e. redshift in light say 10 million years old is much less than that for 10 billion year old light. so the question is should we be using distace or time?

 

[space_buster]

Charts of supernova as a function of red shifts are used to determine the speed of expansion of universe. Adam Riess first looked at the results, he was quite surprised–the expansion of the universe was not decelerating, but accelerating–it was expanding faster and faster! The most likely explanation was that old cosmological constant term of Einstein!

 

[Drakkith]

I have the same problem with redshift. I have seen the graphs and yes speed over distance shows acceleration but change distance for time and you get a deceleration i.e. redshift in light say 10 million years old is much less than that for 10 billion year old light. so the question is should we be using distace or time?

I don't see how your are getting a deceleration. In either time or distance the redshift has been increasing.

 

[John15]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

 

[Drakkith]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

Of course, the closer the galaxy is the less time the light has been in transit to us and the less the force of expansion is on it, resulting in a slower recession velocity. Andromeda is so close that expansion has pretty much zero effect on it and the gravitational force between our galaxies are drawing us closer to gether, resulting in the blueshift we see. Using either distance or time is perfectly acceptable as long as you are consistent and clear about it. Something that is 11 billion years back in time, meaning the light has been traveling for 11 billion years to reach us is NOT 11 billion light years away, it is much further thanks to expansion.

Lets be clear by what we mean by acceleration. The expansion rate has been observed to be increasing over time, meaning that the recession velocity at a particular distance is greater now than it used to be. This is what we mean by "the expansion is accelerating". The redshift itself has always been increasing the further away an object is due to expansion.

 

[John15]

Further clarification would be appreciated.
Granted the object from which light has been traveling from for the 11 billion years is now further away but how do we know it still exists. Also how do you know at what point in that 11 billion years the light was redshifted?
The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

 

[Drakkith]

Further clarification would be appreciated.
Granted the object from which light has been traveling from for the 11 billion years is now further away but how do we know it still exists.

It doesn't exist NOW as we see it, as it has had 11 billion years to change. It may have merged with another galaxy or something, but the matter that makes it up definitely still exists. Matter, mass, and energy do not simply disappear for no reason.

Also how do you know at what point in that 11 billion years the light was redshifted?

The light has been redshifted over the entirety of its journey by the expansion of space. There is no single event that caused the redshift.

The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

No, we look at the recession velocity of galaxies at many different distances from us, since doing so allows us to look at many different points in the past and observe how the universe was back then. I think a single galaxy has such a small increase in recession velocity over 100 years that it is not capable of being measured. Even if we can, we have not been able to measure redshift of galaxies accurately enough for more than a few decades if that. I believe it has only been within the last decade or so that we have been able to measure the velocity of nearby stars to within a few meters per second relative to us.

 

[GeorgeDishman]

Further clarification would be appreciated.

The expansion rate has been observed to be increasing over time, by this am I correct in assuming that a certain galaxy has been watched over say the last 100 years and the redshift is now greater than it was then as this is obviously the only way to be certain that redshift is increasing, comparing 5 and 10 billion year old light cannot in any way be accurate.

Actually, measuring the wavelength of spectral lines is one of the most accurate measurements possible in astronomy. The ratio of the wavelength received to that transmitted is the "redshift".

The following are 'toy' figures intended to illustrate the method and aren't particularly accurate. Suppose we look at three galaxies which are at distances of 100, 200 and 300 million light years and the light from them has been redshifted by 1.1%, 2.1% and 3.0% respectively. We can deduce that the light was "stretched" by 1.1% in the last 100 million years, by 1.0% from 200 to 100 million years ago and by 0.9% in the preceding 100 million years (from 300 to 200). From those we can say that the rate of expansion is increasing by 0.1% per 100 million years.

Does that help explain how a set of specific redshifts can be used to determine not only the current rate but also the history of the rate? In reality of course we have thousands of results each adding a point to the graph as well as a theoretical curve which fits very well.

 

[Naty1]

You guys agonizing about acceleration might take a look here at the 'deceleration
parameter' , q, ...and note the accompanying diagram:

http://en.wikipedia.org/wiki/Hubble_expansion#.E2.80.98Ultimate_fate.27_and_age_of_the_universe

In a link:

... observations of distant type Ia supernovae indicate that q is negative; the expansion of the universe is accelerating.

Type Ia are a 'standard candle' ...a fixed brightness from which we can calibrate things...becuase the fixed brilliance is related to distance...roughly like car headlights from the same model car at varying distances...

 

[SHISHKABOB]

The highest redshift is the CMB at 13.5 billion years, andromeda at 2.5 million years is blueshifted, the further back in time you go the higher the redshift therefore the closer in time the lower the redshift.

just so you know, Andromeda is blueshifted because it is moving towards us due to the gravitational forces between the Milky Way and Andromeda. At small distances, things like gravity are much more influential than the expansion of the Universe.

It's like what if the Moon had a charge of +1C and the Earth had a charge of +1C too. There'd be a repulsive force between the Earth and the Moon due to electrostatic forces, but the force due to Gravity is much stronger and so has much more influence.

 

[John15]

George the first explanation I have seen along those lines, I can see how the accumulation can work, it still assumes though that space is expanding i.e. if light was redshifted as it left the 300 million light year galaxy with the full 3% redshift because it was moving faster then then the redshift from the closer galaxies would be smaller if there had been a deceleration in that time with no expansion of space. Your explanation seems to rely on the expansion of space rather than the movement of the galaxies. Is redshift caused by the expansion of space or the movement of the galaxies?
Also redshift must be relative to the moving bodies i.e. take 3 bodies moving in the same direction along the same axis 1 moving at 100 2 (middle) moving at 70 3 moving at 40. These are all moving apart in the same direction and I think I am correct in saying the redshift from 2 - 3 and 2-1 would be the same.
I am not disputing redshift just questioning if the interpretation is correct and all other possible causes have been checked and ruled out, possibly because I cannot see how space can just materialize out of nowhere unless it worked out like our tectonic plates, maybe the great attractor is an area of dissapearing space.
Regarding andromeda, the BB theory relies at least in part on the fact that galaxies seem to be moving apart so they must have been closer together in the past, of course if andromeda is moving towards us now then by the same reasoning it must have been further away in the past which is a bit contradictory.

 

[bapowell]

andromeda is moving towards us now then by the same reasoning it must have been further away in the past which is a bit contradictory.

The big bang theory is consistent with a uniform and isotropic expansion of the universe on sufficiently large scales where the energy density itself is close to uniform and isotropic. However, not all galaxies are at rest with respect to this expansion (simply being carried along with it), nor should they necessarily be. Interactions with other galaxies, as well as small perturbations during galaxy formation, can lead to non-negligible galactic velocities relative to the expansion (called peculiar velocities.) So, in fact, Andromeda's motion towards our galaxy is perfectly consistent with a full consideration of gravitational dynamics that includes the effects of local structure. As shishkabob says above: the local interaction dominates the tendency for Andromeda to move along with the expansion of the universe. It is not even a bit contradictory.