# Accelerating universe

1. Jul 29, 2013

### azerbajdzan

Universe appears to be expanding at an increasing rate.
What measurements were performed to support this idea?

My current understanding is that in general redshift of distant stars/galaxies is increasing with time.

So to me the measurements may look like this:
We choose one star, measure its redshift, say it would be 1 Hz.
Then one year later we measure the redshift of the same star and it would be, say, 2 Hz.
We perform the same measurements on many stars and if most of them have increased their redshift it suggest that in general the universe expansion increases.

Is it so or my understanding is not correct?

2. Jul 29, 2013

### phinds

The redshift measurements are taken on numerous stars over a wide range of distances.

Redshift is not the only evidence for acceleration. The topic is discussed here VERY frequently. Do a forum search for (for example) "evidence for accelerating universe"

3. Jul 29, 2013

### azerbajdzan

To me the redshift at particular time only means that universe is expanding not that it is expanding at increasing rate.
The evidence of accelerating expanding would be if redshift increases with time. But it looks to me that the difference between redshift in, say, 2012 and in 2013 do not differ to much for it to be measurable. Or is it measurable?

What is other evidence of acceleration?

4. Jul 29, 2013

### phinds

You are completely missing the point. Taking red-shift measurements at different distance is taking them at different times. The light from a galaxy 1 billion light years away was emitted 1 billion years ago. The light from a galaxy 10 billion light years away was emitted 10 billion years ago.

Which part of Do a forum search for (for example) "evidence for accelerating universe" did you not understand ?

5. Jul 29, 2013

### Mordred

As Phinds pointed out we do not rely on one method.

https://www.physicsforums.com/showpost.php?p=4447058&postcount=6

my signature has links with further information. I recommend "Expanding confusion" by Lineweaver and Davies found under the cosmology101 link of my signature. The cosmocalc is also a handy tool to understanding.

6. Jul 29, 2013

### Landrew

Is it really accelerating, or is that how it would appear when you are looking over a vast expanse of expanding space? Since space itself is expanding, it stands to reason that the longer your perspective across it, the more rapidly it would appear to be expanding.

Objects are not really moving away from each other of course, the space between them itself is growing larger. This in itself presents a strange problem. If the earth exists in a narrow "Goldielocks Zone" in it's orbit around the sun, where life could not exist were it much nearer or much farther from the sun. If fossil evidence suggests that life has existed on earth for several billion years, how did the earth manage to stay within the narrow "zone of life" for all that time? Would not the expansion of space have caused a decrease in solar radiation reaching the planet? What are the explanations for this?

7. Jul 29, 2013

### Mordred

The articles I posted cover that, expansion doesn't occur in gravitationally bound regions. Instead it only occurs between galaxies that are not gravitationally bound. Its not acceleration as per se there is no inertia. As you say its simply the amount of space increasing.

8. Jul 29, 2013

### azerbajdzan

I understand it... and I read many articles about it... on wikipedia and others. But this is question that I still do not understand, that is why I am asking.

I understand this. But then they say, that more distant objects have bigger redshifts. More distant means more back in time. So I explain it the opposite way... the more back in the time the more the red shift is the bigger the expansion was... i.e. the universe expansion is decreasing.

That is my logic... I sure that I misunderstood something... and asking where I made a mistake...

9. Jul 29, 2013

### Mordred

perhaps this tool will help, lightcone calculator first link of my signature. I chose a 30 step and column selection of the values I wish to see. The calc has other selections I chose to turn off.

From this you can the redshift vs time aspect to distance relation, the calculator can also graph the results but due to scale of this calculation it wouldn't show much without rescaling vertical and horizontal values. the thread at the top has numerous examples some showing the graph for a better visualization.

$${\small\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.92&0.693&0.307\\ \hline \end{array}}$$ $${\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline S&a=1/S&z&T (Gy)&R (Gly)&D (Gly)&D_{then}(Gly)&a'R_{0} (c) \\ \hline 1090.000&0.000&1089.000&0.0004&0.0006&45.332&0.042&21.02\\ \hline 739.062&0.001&738.062&0.0007&0.0012&45.031&0.061&16.62\\ \hline 501.112&0.002&500.112&0.0013&0.0022&44.654&0.089&13.29\\ \hline 339.773&0.003&338.773&0.0025&0.0040&44.184&0.130&10.71\\ \hline 230.379&0.004&229.379&0.0046&0.0072&43.602&0.189&8.69\\ \hline 156.206&0.006&155.206&0.0084&0.0130&42.888&0.275&7.08\\ \hline 105.913&0.009&104.913&0.0153&0.0235&42.012&0.397&5.79\\ \hline 71.813&0.014&70.813&0.0277&0.0423&40.943&0.570&4.75\\ \hline 48.692&0.021&47.692&0.0501&0.0759&39.639&0.814&3.89\\ \hline 33.015&0.030&32.015&0.0902&0.1363&38.052&1.153&3.20\\ \hline 22.386&0.045&21.386&0.1621&0.2445&36.120&1.614&2.63\\ \hline 15.178&0.066&14.178&0.2911&0.4383&33.771&2.225&2.16\\ \hline 10.291&0.097&9.291&0.5223&0.7851&30.918&3.004&1.78\\ \hline 6.978&0.143&5.978&0.9361&1.4037&27.455&3.935&1.47\\ \hline 4.731&0.211&3.731&1.6741&2.4969&23.266&4.918&1.22\\ \hline 3.208&0.312&2.208&2.9777&4.3736&18.248&5.688&1.03\\ \hline 2.175&0.460&1.175&5.2154&7.3341&12.398&5.700&0.90\\ \hline 1.475&0.678&0.475&8.7894&11.1153&6.042&4.097&0.88\\ \hline 1.000&1.000&0.000&13.7872&14.3999&0.000&0.000&1.00\\ \hline 0.681&1.468&-0.319&19.7042&16.2016&4.910&7.207&1.30\\ \hline 0.464&2.154&-0.536&26.0846&16.9288&8.515&18.346&1.83\\ \hline 0.316&3.162&-0.684&32.6380&17.1800&11.040&34.912&2.65\\ \hline 0.215&4.642&-0.785&39.2497&17.2617&12.776&59.303&3.87\\ \hline 0.147&6.813&-0.853&45.8801&17.2877&13.963&95.126&5.67\\ \hline 0.100&10.000&-0.900&52.5163&17.2961&14.772&147.715&8.33\\ \hline 0.068&14.678&-0.932&59.1545&17.2987&15.323&224.908&12.22\\ \hline 0.046&21.544&-0.954&65.7934&17.2994&15.698&338.212&17.93\\ \hline 0.032&31.623&-0.968&72.4323&17.2998&15.954&504.520&26.32\\ \hline 0.022&46.416&-0.978&79.0713&17.2998&16.129&748.627&38.64\\ \hline 0.015&68.129&-0.985&85.7103&17.3000&16.247&1106.926&56.71\\ \hline 0.010&100.000&-0.990&92.3494&17.2999&16.328&1632.838&83.24\\ \hline \end{array}}$$

10. Jul 29, 2013

### jackmell

Why isn't anyone mentioning the supernova data to support accelerated expansion?

11. Jul 29, 2013

### phinds

You have a LOT of company in thinking that --- it seems to be the first thought of a great many people when first looking at it and it DOES seem reasonable at first. I think Mordred's post will help you understand.

12. Jul 29, 2013

### Mordred

it is mentioned the cosmic distance scale of the first article I linked

https://www.physicsforums.com/showpost.php?p=4447058&postcount=6 see the bottom section of that article.

here is a cut and paste from it

Cosmic Distance ladder, also known as Extragalactic distance scale. Is easily thought of as a series of different measurement methods for specific distance scales. Previous in the article we discussed the various forms of Redshift. These principles are used in conjunction with the following methods described below. Modern equipment now allows use spectrometry. Spectrographs of an element give off a definite spectrum of light or wavelengths. By examining changes in this spectrum and other electromagnetic frequencies with the various forms of shifts caused by relative motion, gravitational effects and expansion. We can now judge an objects luminosity where absolute luminosity is the amount of energy emitted per second.

Luminosity is often measured in flux where flux is

$$f=\frac{L}{4\pi r^2}$$

However cosmologists typically use a scale called magnitudes. The magnitude scale has been developed so that a 5 magnitude change corresponds to a differents of 100 flux.
Rather than cover a large range of those distance scales or rungs on the ladder I will cover a few of the essential steps to cosmological distance scales. The first rung on the ladder is naturally.

Direct measurements: Direct measurements form the fundamental distance scale. Units such as the distance from Earth to the sun that are used to develop a fundamental unit called astronomical unit or AU. During the orbit around the sun we can take a variety of measurements such as Doppler shifts to use as a calibration for the AU unit. This Unit is also derived by a method called Parallax.

Parallax. Parallax is essentially trigonometric measurements of a nearby object in space. When our orbit forms a right angle triangle to us and the object to be measured
With the standardized AU unit we can take two AU to form the short leg. With the Sun at a right angle to us the distance to the object to be measured is the long leg of the triangle.

Moving Cluster Parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster.

Stellar parallax is the effect of parallax on distant stars . It is parallax on an interstellar scale, and allows us to set a standard for the parsec.

Standard candles A common misconception of standard candles is that only type 1A supernova are used. Indeed any known fundamental distance measurement or stellar object whose luminosity or brightness is known can be used as a standard candle. By comparing an objects luminosity to the observed brightness we can calculate the distance to an object using the inverse square law. Standard candles include any object of known luminosity, such as Cepheid’s, novae, Type 1A supernova and galaxy clusters.

Last edited: Jul 29, 2013
13. Jul 29, 2013

### Mordred

just as side note, on the accelerating expansion rate lets take a simple math expression

x=x+expansion%. set x= distance

its easy to see why its considered an accelerating expansion.

14. Jul 30, 2013

### Lino

azerbajdzan, The key thing that red-shift shows us is that the speed at which an object moves away from us is proportional to its distance from us - it doesn't have to be accelerating faster. Think of a car moving away from you (or traffic lights), it is going 10mph after 1 second, 20mph after 2 seconds, 30mhp after 3 seconds, ... - its speed is getting bigger not its rate of acceleration - the acceleration is constant. The same applies to the stars / galaxies moving away from us - the further from us that they are, the faster they are moving, the greater their redshift - even if their acceleration is constant.

And you're right, if we could measure the change in the red-shift from year to year, we would see it growing. However, the rate of expansion is only (approximately) 1/144% per billion years and (at present) we can't measure red-shift that accurately. That's why, as phinds mentioned we do lots (and lots) of measure at lots of different distances, which equates to doing measures billions of years apart and shows us the different values for red-shift. And yes, the rate of acceleration was less for those more distant objects (because we are looking back in time), but only by relatively small amounts. In terms of understanding red-shift, you can ignore this small part initially.

If you want to know more about the changing rate of acceleration due to expansion, there are special objects (a specifc type of supernovea as jackmell mentioned) that do allow us to gather even more information because we know more about them, and it is these and other evidence that shows us that the rate of acceleration is growing and not just the speed. You probably know that red-shift was identified in the 1950's by a guy called Hubble, but you might not be aware that it was only (approximately) 10 years ago that a guy called Perlmutter (and others) identified the changing / increasing rate of acceleration. If you do the search that phinds suggested, you will find plenty of material related to this.

15. Jul 30, 2013

### George Jones

Staff Emeritus
Not for all objects.

16. Jul 30, 2013

### Lino

Thanks for reminding me of this George, very much appreciated. Would you think that the OP should ignore this for the moment and focus on getting to grips with basic (I don't know what other word to use) red-shift first?

17. Jul 30, 2013

### azerbajdzan

All current observations are for me not enough convincing to say that universe is expanding at increasing rate. And then think up some dark matter and dark energy to support this idea. More likely it is that there is no dark matter (except black holes, faint stars and similar common objects) and no dark energy... but that we do not explain what we see by observation correctly and that another "Einstein" is needed to explain why it is so without using any "dark energy" in the theory.

Do not understand it that I am presenting myself in the position that I am able to explain these things correctly.
No, I have no idea too. I am just skeptical about current explanations using "dark energy".

18. Jul 30, 2013

### phinds

Well, good luck with that.

Again, you misunderstand completely. Dark matter has nothing to do with expansion.

No, none of those can possibly be dark matter. You clearly do not understand the characteristics of dark matter.

Your problem throughout seems to be that for both dark energy and dark matter (which have NOTHING to do with each other) you are simply ignoring the direct results of empirical observations.

19. Jul 30, 2013

### azerbajdzan

I have not said that dark matter has something to do with expansion.
It is dark energy that causes expansion by today's physics.
I just mention dark energy and dark matter together as an examples that when people are not able to explain something then they invent some terms/ideas to support the theory. But the reality could be that there is nothing such as dark energy or dark matter and that it is only the incomplete theory.

Maybe today's theory needs the same step forward like it was step from Newton to Einstein. So maybe another "Einstein" in the future would be able to explain what we see by theory where there would not be any dark energy or dark matter.

20. Jul 30, 2013

### WannabeNewton

This thread has derailed into nonsense to be honest. There is a wealth of information on dark energy that you can read up on through the internet; you should do so before making grandoise claims about its non-existence/invalidity.

21. Jul 30, 2013

### azerbajdzan

Many prominent physicists admit that it is possible that "dark energy theories" could be wrong, although for now it is the most satisfying theory. (Like Newton's theory was/is imprecise in some situations, so in strict sense it was/is wrong in general)

Example:
Perihelion precession of Mercury... if there were no Einstein some physicist could make up, say, "transparent energy" as an explanation why perihelion precession of Mercury does not comply with Newtonian mechanics. This theory could describe observation of Mercury precisely using some properties of hypothetical "transparent energy". But then there would come Einstein and would say... there is no need for "transparent energy", look, here is my General Theory of Relativity which explains the situation with Mercury and it does not need any existence of "transparent energy" and in fact no "transparent energy" exists.

The same can happen with "dark energy".

22. Jul 30, 2013

### Staff: Mentor

Physical models are always "the best thing we know today to explain observations".
Those observations are sometimes model-dependent, sometimes not.

No. Compared to simple extrapolation based on the current expansion rate, they have less redshift. Which means expansion was slower in the past.

23. Jul 30, 2013

### azerbajdzan

So bigger or less? Your post confused me even more.

This is quote from this link http://www.setterfield.org/000docs/redshift.htm [Broken]:
"Edwin Hubble showed in 1929 that the more distant the galaxy, the larger this "redshift"."

Last edited by a moderator: May 6, 2017
24. Jul 31, 2013

### marcus

There is some truth in what you say. I'm sleepy now and can't talk. Please check back here in 10 or 12 hours, give me time to get some sleep. We may find some things we can agree on.

Also part of the confusion has to do with imprecision of language. Often, in English, when someone says "growth rate" they mean the PERCENTAGE rate of increase. Like 2% increase per year. But "rate" can also mean actual SPEED. It's different.

A distance, say the height of a tree, can increase at a CONSTANT (percentage) rate and increase at a faster and faster speed. It can even grow at a gradually DECLINING (percentage) rate and still grow at an increasing speed (measured in centimeters per year).

So that is a problem with ordinary common spoken language. It is not as clear as mathematics. You need to be more clear about what you mean by "acceleration", and what you think other people mean by "acceleration".

The current rate of distance expansion in the universe is 1/144% per million years, according to latest measurment. This is believed to be decreasing and it is expected to continue decreasing in the foreseeable future. According to standard cosmic model it will decrease to 1/173% per million years in the very long term.

This decrease in the expansion rate is so slow that if you could watch a particular large distance grow for a long time you would see it get bigger at an increasing speed.

If you are interested in getting clear about cosmic expansion and what people mean by "acceleration" in this context, please give me a chance to get some sleep and check in here later.

I think you are right to be skeptical about "dark energy". The effect might not be caused by an actual "energy" but simply by a constant term in the equation that governs cosmological distances. It has not been proven that it is due to any actual energy.

25. Jul 31, 2013

### Staff: Mentor

For more distant galaxies, the redshift is larger - this just means that the universe expanded all the time.
For more distant galaxies, the redshift is not as large as you would expect for a constant expansion - this effect is way smaller than the first one, and it corresponds to the acceleration of the expansion.

Last edited by a moderator: May 6, 2017