FUGE and Redshift

I have written before about the OE Curve and redshift, but it was blended together with consideration of someone else’s theory about the universe so it’s not as clear as I would have liked.  So, I’ll have another go at it.

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There are (at least) two ways to think about redshift.

A photon which is emitted by a source that is moving away at a sufficiently high speed can be observed to be redshifted.  That’s not to say there isn’t redshift for slow (non-relativistic) light sources, it’s just that the redshift in that case is so small as to be unobservable.  This form of redshift is effectively the same as Doppler shift, but requires some extra thought due to relativistic effects (see relativistic Doppler shift).

A photon that is emitted by a source that is stationary (at rest relative to the Hubble flow) or “comoving” relative to the observer, if sufficiently distant from that observer, can also be observed to be redshifted.  Again, the redshift is there, irrespective of the distance under those conditions, but at relatively short distances it’s too small to be observed.

(There is a third source of redshift, which is due to gravity.  It’s actually similar to the redshift due to cosmological expansion in a way, but at a different scale, since both involve deformation of space along the path of the photon.  Redshift due to gravity occurs when the photon is moving out from a gravitational field.)

I want to address redshift due to cosmological expansion.  First think about a photon moving through space in a given direction.  It has a wavelength determined by the speed of light, since a photon is a wavelet oscillating with a frequency determined by its energy (which is related to its colour and associated temperature).  That wave traces a path through space like this:

Say we stretch the original space illustrated above by a factor of two (looking at only the first full wavelength), we now get this:

The speed of light remains constant, so what we have here is a photon with twice the wavelength and half the frequency.  It’s an extreme case of redshifting.

To determine the redshift, z, we take the second wavelength, λ_now, subtract the original wavelength, λ_then, and divide by the first wavelength:

z=(λ_now-λ_then)/λ_then=(λ_now/λ_then)-1

Or,

z+1=λ_now/λ_then

Note that this equation is the same as standard redshift, where now=obsv and then=emit.

In a FUGE universe, the radius of the universe at a given time t will be ct.  The wavelength of an arbitrary photon at that time will be some fraction of that radius, so we can consider a photon of wavelength λ_then=ct_then/B.  After a period of expansion that same photon (now) will have the wavelength λ_now=ct_now/B.  Substituting this into the above and we get:

z+1=t_now/t_then

Alternatively, we can think about scale factor over time.  In a FUGE universe, a(t)=ct/ct₀ where is a reference time (usually now, but it doesn’t have to be in the case).  Note that cosmological redshift is given by

z+1=a_now/a_then

So using the scale factor equation above:

z+1=(ct_now/ct₀)/(ct_then/ct₀)

z+1=t_now/t_then

Alternatively, we can think about the OE curve, for which the equation is x'=(ct₀-x).x/ct₀.  Note that this could be confusing, because in all the OE curve articles in which I clarify that x=ct, I use t to refer to the time elapsed between emission time of the photon (or when it was in a particular location) and the observer.  The t used above (and in the redshift literature more generally) is a reference to the age of the universe (for which I use æ).  Also, in the OE curve equation, rather than being just a reference time, t₀ is specifically the current period of time since the beginning of the universe, or perhaps less confusingly t₀ is the current age of the universe, so t_now=t₀.

I do note, in Mathematics for Taking Another Look at the Universe, that in the OE curve equation it would be more accurate to say that it would be more accurate to use Δt and Δx, so Δx=c.Δt and thus

x'=(ct₀-Δx).Δx/ct₀=(ct₀-cΔt).cΔt/ct₀

I’ll use this notation for as much clarity as is possible under the circumstances, noting that Δt is referring to the delta between now and then, so Δt=t_now-t_then=t₀-t_then, and Δx is the distance a photon in static space would travel in that time.

Consider, hypothetically, that redshift might be due to the difference between the actual separation crossed and the distance that the photon needed to travel to cross that separation, or:

z=Δx/x'

Note that what this is effectively doing is comparing the distance between two comoving locations (emitter and observer) at two times (relative to the observer), time of emission and time of observation.

Substituting in the OE curve equation:

z=Δx/((ct₀-Δx).Δx/ct₀)

z=ct₀/(ct₀-Δx)

Noting that Δx=c.Δt:

z=t₀/(t₀-Δt)

And then noting that Δt=t₀-t_then and t₀=t_now:

z=t_now/t_then

This isn’t precisely the same as above, but for sufficiently high values of z, z+1≈z, so it’s a good approximation.

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The problem, of course, as identified in an early article about redshift, is that the value for CMB redshift calculated this way is in the order of z≈36,000 (assuming t_d=380,000 years).  The accepted value is z≈1100.

However, this value is based on the assumption that the universe expanded such that H(t)=2/3t since decoupling/recombination (at t=t_d) which in turn would mean that z=(t_d/t₀)^⅔.  Note that 36,000^⅔≈1100.

I think that, if the FUGE universe model were to be correct, the redshift value associated with the CMB could still be correct, but the timing would be different, by a factor of ~32, so decoupling/recombination would have happened at t_d≈12.5 million years (which would have little impact on the oldest star – Methuselah – which is calculated to be 14.46 billion years old).

From what I can establish from looking at widely available information on the topic, the key event of decoupling/recombination is that the universe was sufficiently cool for the electrons to be captured by atomic nuclei.  This is 3000K, and the remnant radiation (in the form of the CMB) is currently 2.7K, so this suggests a redshift value of z=1100.  Then they have worked backwards, assuming the Standard Model (which is entirely reasonable) to say that the universe was at this temperature when it was 380,000 years old.

In the FUGE model, however, decoupling/recombination would have happened at 2.7/3000*13.8×10⁹=12.5 million years.

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There’s another aspect of redshift, which I haven’t really mentioned above, which is the shift in colour.  The colour of a photon is related to its wavelength – red photons have a wavelength in the region 625-740 nm.  Blue photons have a wavelength in the region 450-495nm.  You can see, therefore, that as a photon’s wavelength is increased, its colour is changed in the red direction.

If the colour of a photon is pushed even further, it leaves the visible spectrum into infrared and eventually into the microwave spectrum (pushing it further you could even get radio waves).

We can think of the CMB as being a blackbody radiation spectrum associated 2.75K and indeed the correspondence is nigh on perfect with that (source):

The find the peak of this spectrum, we can use Wein’s displacement law, λ_max=b/T, where b(=2.897×10^-3 m.K) is the constant of proportionality and T is the temperature in kelvin.

Given the current temperature and the temperature at decoupling/recombination (t_d=3000K), we can work out the wavelengths:

λ_{max_CMB}(t₀)=2.897×10^-3/2.75=1.05×10^-3m

λ_{max_CMB}(t_d)=2.897×10^-3/3000=966×10^-9m

Note that this latter wavelength actually corresponds with infrared and the first, unsurprisingly, is in the microwave spectrum.

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There is another way to arrive at a value of redshift in a FUGE universe.  This is via equations in notes from Oxford university (3.c. (Solution)), where it is stated that H(t_d)=H₀.1000√1000 and that (t_d/t₀)^⅔=1/1000.  If so, then it follows that (H(t_d)/H₀)^⅔=1000=(t₀/t_d)^⅔, and thus H(t_d)/H₀=t₀/t_d.  Since t₀=t_now and, in this instance, t_d=t_then, we have:

z≈t_now/t_then=H(t_then)/H(t_now)

Again, this should come as no surprise as, in a FUGE universe, H(t)=1/t.  However, if there were any direct evidence that, at decoupling/recombination, the value of the Hubble parameter was in fact H(t_d)≈ 7700, then we’d have another way to work out that z_CMB≈1100.