Expansion of the universe

GeorgeDishman

It is a curve-fit to a large number of individual point measurements. When we look at a distant galaxy and see a supernova in it, we can measure both the redshift of the galaxy or the supernova and also the brightness. That gives us one point on the curve. Hunderds of such measurement pairs give us the relationship which we can then fit by adjusting parameters in the equations.

No, it's like making lots of individual measurements of skin colour versus sugar content of a particular type of apple at different stages of ripening and plotting a graph. Going back to your original question, the graph then lets you estimate the relative difference in sugar content of two apples based on the difference in skin colour even if neither is fully ripe. Similarly, we could tell how far apart two distant supernovae were from their redshifts and our graph of the scale factor.

marcus

Friedmann did not "employ Newtonian gravitation..." Einstein's GR equation came out in 1915 and Friedmann was using it.
Friedmann equation is a simplification of the Einstein GR equation derived by making the simplifying assumption that the universe is approximately uniform (matter evenly distributed, more or less). If it is close enough to uniform, at very large scale, e.g. with same number of galaxies per unit volume everywhere, then you can TREAT IT AS IF IT WERE perfectly uniform and then General Rel geometry simplifies enormously and you get Friedmann equation.

You also get a concept of "universe time" or "Friedmann time" as it is sometimes called, and you get a concept of the scalefactor a(t) that shows how distances increase with time. The Hubble rate H(t) is defined to be a'(t)/a(t). That is the time derivative of a(t) divided by a(t) itself. It is is always changing and in the past has fallen off very rapidly. It is still changing but more slowly now.

Ah! I see you use R(t) for the scale factor, instead of a(t). Both notations are in use.

No, the facts do not require exponential growth of R(t). Because H(t) is not constant.
When people write H₀ they mean the CURRENT value of H(t) that it happens to have at present. If H(t) were constant, then you would get an exponential solution for R(t).
But it isn't constant. For about half the history of expansion the R(t) curve has been convex (decreasing slope) and then it had an inflection point and became concave (increasing slope). Far in the future it may have approximately an exponential growth shape. But if you are talking past and present then what you say runs against what's been learned so far.

Drakkith

Think of it this way. Because the photon cannot lose speed, it MUST lose momentum by losing energy, and since the photon's energy and momentum is determined by it's frequency, it redshifts to a lower frequency. Now, realize that the photon isn't doing anything itself, it is merely a consequence of your frame of reference being different than the frame of reference that emitted the photon.

Yes. A lower frequency has less energy than a higher frequency. Just like a moving object has less kinetic energy when viewed from something moving away from it's source, the photon will have less energy when viewed from something moving away from it.

mufa

Yes,I understand this.My petition is other:Why photon should not be an ultra small particle behave as a wave,like the electron does? Why we claim that it is a wave with particle properties and not the opposite,such as the electron?This model,as suggested above by elias2010 not leads to dark energy.

MDEarl

Thanks Marcus, you’re right. I read somewhere that Friedmann was an early proponent of Einstein’s work and assumed the “classical derivation” of his formulas came first. Not so.
Also, Sorry I used R instead of a, I hope you don’t mind me continuing it, I get confused using a for a distance.
The Newtonian reference has come up in other conversations that I have had…my claim was (is) that G immediately calls in Newton’s law…not necessarily directly, but regardless of the situation, bringing G into an equation brings in Newton. To my knowledge, G is strictly empirically derived from Gmm/r^2. If you can find an exception, please let me know… I’ve been playing with that for a while now, and have not been convinced otherwise yet.
I am a skeptic, particularly regarding complex theories. I have always had a problem with the FLRW (standard) model because of that. I’m retired (like you) and question complicated science. Darwin, Einstein, Wegener, etc. addressed seemingly complex situations with simple overriding laws, they didn’t amend the accepted theories. Every time I hear about dark matter and energy, or bringing back the cosmological constant, parallel universes, etc., I cringe. It reminds me of the “luminiferous ether” which we students used to laugh at in school (LE was way before my time, though). Everyone would make fun of the massless, invisible undetectable medium, very similar to dark stuff today, and wonder “what was going through their minds?”. I’m sure there were probably experiments that seemed to confirm its existence before Michelson/morley and Einstein came along.
It is my opinion that cosmology today needs to somehow go back to basics, rather than adjust the standard model. An exponentially accelerating universe produces amazing results…I have become convinced of its validity. I’ve done a lot of calculations over the past 3 years.
Some tenets of an expon. acc. model (which may differ from standard)

Density of universe is constant (both mass/energy and spacial volume increase correspondingly)
Density of local situations such as a mass (earth) surrounded by expanding space produces Newton’s law (mathematically)
Time is invariant , (I know this seems to have relativistic problems, but relativity works out in the model)
Ho is constant.
Cosmological principle ok
A question on the H(t) … I am not familiar with a changing Ho, is this related to the inflation period that is proposed in the standard model? Have there been measurements to confirm that it is changing?
Also, Alchemist mentions an exponential expansion early in this thread… did he really mean exponential?...(e^Hot)?
Thank you very much for your comments.

Lino

Thanks George. (Kinda understand and just the info I was looking for.)

Regards,

Noel.

mufa

De Broglie equation has experimentally confirmed.Why we have to reject the hypothesis photon have a mass instead of the possibility being wrong the relativistic formula for energy?

bapowell

Because experiments have failed to detect a photon mass greater than [itex]10^{-27}[/itex] eV (see http://en.wikipedia.org/wiki/Photon#...on_photon_mass.)

harve

George,I read your posts.Can you explain to me what an elegtromagnetic pulse is? Because I thought that is a wave with only one peak represents a big quantity of photons.

Landrew

My confusion comes from whether objects in space are moving apart, or whether space itself is getting larger. The distinction seems less than clear in most discussions I've seen.

http://en.wikipedia.org/wiki/Accelerating_universe
http://en.wikipedia.org/wiki/Metric_expansion_of_space

Whovian

They're indistinguishable, it's just more convenient to think of it as space expanding.

GeorgeDishman

I've added some context as the quotes are quite old.

A single pulse has a DC component so can't be a simply EM signal in space say. If it was a switched DC signal on a wire, you'd have to look at the components of the Poynting Vector and it all gets complicated, however I understand what you are asking. You can consider instead a rectangular wave with narrow, widely separated pulses and zero average value.

A square pulse is the sum of many sine waves, you can get the pattern by taking a Fourier Transform. For a regular series of pulses there are discrete harmonics while for a single pulse you get a continuous spectrum. Either way, you can then break down each sine wave into numbers of photons by dividing the portion of the pulse energy in that frequency by the energy of a single photon. As you say, ultimately you will get a burst of photons but of a mixture of frequencies. Again, each photon can be thought of as a burst of waves and they overlap to create the macroscopic, measurable, sine wave.

That's very different to what the O.P. was saying, that "a peak [of a sine wave] represents an amount of photons".