I tried calculating redshift in a FUGE universe, and ran into what looked to be a problem.
My logic was as
follows: if we are observing a photon from an event that occurred a time
t ago, then that photon had a proper distance of x'=x.(ct0-x)/ct0
and the event location (at the time of observation) will be x=ct from
the observation location. So for any
arbitrary wavelength of the photon:
1+z=λobserved/λemitted=x/x'=ct0/(ct0-x)
z=ct0/(ct0-x)-1=(ct0-(ct0-x))/(ct0-x)=x/(ct0-x)
z=t/(t0-t)
The problem is that
using this equation, redshift for the CMB does not come out to be what is
generally attributed to it (zCMB=1100). Recall that t0 here is the
age of the universe and t is the transit time for an observed photon
(see most recent posts, particularly Mathematics for Taking Another Look at the
Universe), so for a photon emitted
during recombination, the
event (also confusingly known as decoupling) that led to the CMB when the universe was 380,000 years old, tCMB=(13.8×109-380,000)
years and:
zCMB=tCMB/(t0-tCMB)
zCMB=(13.8×109-380,000)/(13.8×109-(13.8×109-380,000))=36,300
This is 33 times
higher than the standard answer. We can
reorganise the equation above to work out how old the universe must have been
for photons from the CMB to have a redshift of z=1100. Using ꬱCMB=t0-tCMB (and thus tCMB=t0-ꬱCMB):
zCMB=(t0-ꬱCMB)/ꬱCMB=t0/ꬱCMB-1
t0/ꬱCMB=zCMB+1
ꬱCMB=t0/zCMB+1
For zCMB=1100,
we get an ꬱCMB=1.25×106 years. Hm,
it appears that something is not right.
---
The question that
immediately arises, at least for me, is how do we know that the redshift for
photons from recombination is 1100 and how do we know that recombination
happened when the universe was 380,000 years old? And what are the error bars associated with
these values?
The redshift value
is a little rubbery, but is usually quoted as simply 1100, although it’s
probably a bit lower. Rhodri Evans (astrophysicist
and author of The
Cosmic Microwave Background - How It Changed Our Understanding of the Universe) has a blog
post on the CMB redshift
which gives a bit of the history, indicating that the value comes from a
comparison between the temperature of the CMB radiation today and that at the
time of recombination, so:
zCMB=Trecomb/Tnow=3000/2.725≈1100
This equation is a
slight approximation, since it should be z+1=Trecomb/Tnow
and I will be using the non-approximation from now on.
Note also that the
equation does not explicitly rely on the timing of the event. The recombination is thought to have happened
when the universe got sufficiently cool, so the timing isn’t actually key. While Evans does write that “as the Universe
expands, the temperature (..) decreases in inverse proportion to its size. Double
the size of the Universe, and the temperature will halve”, to know when the
temperature of the universe was 3000K, we would have to know what the
temperature was at some other time, what that time was and what the relevant expansion
rate was.
Note that in a FUGE
universe, the radius of the universe is directly proportional to its age (so
currently r0=ct0).
So, noting the inverse relationship, we could simply replace Trecomb
and Tnow with 1/ꬱrecomb=1/380,000 years and
1/ꬱnow=1/t0=1/13.8×109 years. Which gives us … z≈36,300.
So, I still have
questions about timing and redshift and now also temperature.
---
When I expanded my
search to find out how the relevant temperatures are calculated, I stumbled on
what looks to be a variant of the FUGE universe, described in papers that have
been published in what appear to be legitimate journals. This model is referred to more frequently as Flat
Space Cosmology, but there
are also references to “rH=ct models” (presumably those
similar to Melia's, where t is the age of the universe)
and “growing
black hole models”
which seems to describe something akin to the FUGE universe (I disagree with
the terminology but that may just be a matter of perspective).
Espen Haug and
Eugene Tatum (and others) have, in the past half a year, published a number of
papers, for the most part on open archive sites but sometimes in journals (for
example the International Journal of Theoretical Physics). The paper that most attracted my attention
provides a method for calculating temperature, Solving the Hubble Tension by
Extracting Current CMB Temperature from the Union2 Supernova Database (available from HAL open science, which is an
open archive).
I’m not going to
get into whether or not they have actually solved the Hubble Tension, instead I
am going to look at equations in that paper that I have issues with.
The first appears
late in the paper:
The complexity of
this equation does not appear justified, since it resolves to:
Ʊ=2(4π/TP)2/tP
where TP
is Planck temperature and tP is Planck time. This follows from TP=EP/kb
where EP=mPc2 is Planck energy and kb
is the Boltzmann constant, noting that mP=√(ħc/G), so that:
Ʊ=kb232π2G1/2/c5/2ħ3/2=((EP/TP)2.2.(4π)2/c2ħ).√(G/ħc)
Ʊ=((mPc2/TP)2.2.(4π)2/c2ħ)/mP=(mPc2/ħ).2.(4π)2/TP2
Ʊ=(√(ħc/G).c2/ħ).2.(4π/TP)2=(√(c5/ħG).c2/ħ).2.(4π/TP)2
And since tP=(√(ħG/c5):
Ʊ=2.(4π/TP)2/tP=2.923×10-19K-2s-1
This “composite constant” upsilon, which has no other apparent name than the Latinised Greek letter used to denote it, has no other apparent use than in the equation H0=ƱT02, where T0 is the (current) temperature of the CMB. In the paper in which upsilon is introduced, Upsilon Constants and Their Usefulness in Planck Scale Quantum Cosmology, it is derived purely from this simpler relationship. (I should provide a warning here that SCIRP is considered to be a predatory publisher, meaning that there is no peer review for articles which are published after payment. Tatum (who is an anatomic & clinical pathology specialist during the day) indicates that he consulted Dr. Rudolph Schild of Harvard-Smithsonian Center for Astrophysics. Unfortunately, Schild apparently publishes in a fringe (and allegedly predatory) astronomy journal, Journal of Cosmology, of which he is the editor in chief. Tatum has published in that journal at least twice. That all said, if the mathematics is correct, it is correct irrespective of where it has been published, even if the author paid to have it published.)
Earlier in Solving the Hubble Tension by
Extracting Current CMB Temperature from the Union2 Supernova Database, there was something that really attracted
my attention, an equation for the current CMP temperature T0:
Once again however,
this can be simplified. Note that RH=c/H0
is the Hubble radius which, in a flat universe, is equal to RH=ꬱ.lP,
and that lP=√(ħG/c3) so:
T0=ħc/(4π.kb.√(ꬱ.lP.2.lP))=ħc/(4π.(mPc2/TP).lP.√(ꬱ.2))
T0=ħc/(4π.(√(ħc/G).c2/TP).√(ħG/c3).√(2ꬱ))
T0=TP/(4π.√(2ꬱ))
Meaning that the only
equation in which upsilon is used, H0=ƱT02,
resolves to:
H0=ƱT02=2.(4π/TP)2/tP.(TP/(4π.√(2ꬱ))2
H0=1/(ꬱ.tP)
This is precisely
what one would expect from a flat universe.
There is a slightly
different approach, just using RH=Ho.c, but it is
messy.
This messiness can be alleviated if we work with T02,
so that:
T02=(ħc/(4π.kb.√((c/Ho).2.lP)))2=ħ2c2/((4π)2.kb2.((c/Ho).2.lP))
T02=ħ2c2/((4π)2.(mPc2/TP)2.((c/Ho).2.√(ħG/c3)))
T02=ħ2c2/((4π)2.(mP2c4/TP2).((c/Ho).2.√(ħG/c3)))
T02=ħ2c2/((4π)2.((ħc/G).c4/TP2).((c/Ho).2.√(ħG/c3)))
Then rationalising
and rearranging
T02=√(ħG/c5).Ho/(2.(4π)2/TP2)=tP.Ho/(2.(4π)2/TP2)=Ho/Ʊ
So, of course, H0=ƱT02.
---
A potential
critique, if one were to only consider Solving the Hubble Tension by
Extracting Current CMB Temperature from the Union2 Supernova Database, is that all Haug and Tatum have done here
is shuffle numbers around in order to hide H0 in T0,
and then created a new constant (Ʊ) which does nothing more than reveal
the previously hidden H0.
While that may appear to be the case, such critique ignores the fact
that T0 is a measured value, specifically the temperature of
the CMB today or 2.72548±0.00057 K (from 2009 but seemingly still most commonly used). As Tatum showed in Upsilon Constants and Their
Usefulness in Planck Scale Quantum Cosmology, upsilon can be used to extract H0
from the CMB temperature:
ƱT02=(2.923×10-19K-2s-1).(2.72548K)2=2.171×10-18s-1
Noting the km to
Mpc ratio, 3.086×1019km/Mpc, we find:
ƱT02=66.99km/s/Mpc≈H0
This would
correspond to a FUGE universe which is 14.60 billion years old.
---
The equation for T0
was presented in a slightly different format in an earlier paper by Tatum,
Seshavatharam and Lakshminarayana The
Basics of Flat Space Cosmology:
Why the third term is there is beyond me, given the parallels between the second and the fourth. The first two terms are a modification to the Hawking radiation temperature equation, as Tatum acknowledges in Upsilon Constants and Their Usefulness in Planck Scale Quantum Cosmology, (noting that kb is the same thing as kB). The standard Hawking radiation temperature equation:
The implication here
is that Tatum et al. are conceptually equating the temperature of the CMB to
the temperature of a black hole with the same radius, but with a different
value for mass – which doesn’t make sense because with a different mass you are
no longer talking about a black hole. Alternatively, they are effectively scaling
that temperature (and the expression of mass in their equation as being less
than that of an equivalent Schwarzschild black hole is misguided).
This leads to an
intriguing notion and the potential for a different approach, in terms of establishing
redshift.
Consider the Hawking
radiation temperature for a black hole with the mass and radius of the
prevailing Hubble sphere, now and at the time of recombination (when the CMB
was generated). Recall that, per
Carroll, “a spatially
flat universe remains spatially flat forever” and “the corresponding
Schwarzschild radius … equals the Hubble length”. As a consequence, the radius of the universe
now is rH-0=c/H0 and, at recombination, it would
have been rH-recomb=c/Hrecomb, where Hrecomb
is Hubble parameter for that time.
According to Andrei
Starinets’ General Relativity and Cosmology solution notes, the Hubble parameter at recombination (note
that the event is referred to in the notes as “decoupling”) is H(td)=Hrecomb=
H0.1000√1000. Noting that
Schwarzschild radius is directionally proportional to mass, and Hawking
radiation temperature is inversely proportional to radius, we have (per the equation
above from Rhodri Evans, without the approximation):
zCMB+1=TH-recomb/TH-0=rH-0/rH-recomb=(c/Hnow)/(c/
Hrecomb)=Hrecomb/H0
zCMB+1=1000√1000=31,600
Hm, still not right but it is closer to my answer but given that a(t0)/a(td)=1000 should be almost certainly thought of as a(t0)/a(td)=103, rather than a(t0)/a(td)=1.000×103. The tutorial notes go on to state that:
So, if we plug in the
values td=380,000 years and t0=13.8×109
years, we get:
Curiously, if a(t0)/a(td)=1100 is used, we get z=36,000.
Note also that the solution
notes do something akin to a pea and cup trick (similar to Tatum’s apparent hiding
and revealing of H0 above).
He presents three equations:
a(td)=a(t0)/1000 … H(td)= H01000√1000
… (td/t0)2/3= a(t0)/a(td)=1/1000
It is not difficult
to see, when these equations are put side by side to see that:
H(td)/H0=10003/2 and t0/td=10003/2
So:
H(td)/H0=t0/td
It is unclear why
this much simpler and, in retrospect, obvious relationship is not used.
We can go further
to show that, since H(td)=Hrecomb, td=t0-tCMB (where tCMB as defined above is how long ago the CMB
was generated) and zCMB+1=Hrecomb/H0:
zCMB+1=t0/(t0-tCMB)=t0/(t0-tCMB)-(t0-tCMB)/(t0-tCMB)+1=tCMB)/(t0-tCMB)+1
And generalising:
z=t/(t0-t)
Which is the result
that I arrived at, using a second approach
---
Another approach is
on the basis of a FUGE universe using Evan’s equation zCMB=TH-recomb/TH-0.
In a FUGE universe,
the current mass is M(t0)=(mP/2)t0/tP
and at any time t ago, M(t)=(mP/2).(t0-t)/tP. Noting that mass is directly proportional to Schwarzschild
radius and temperature is inversely proportional to radius, the equation above
becomes:
z+1=Trecomb/Tnow=M(t0)/M(tCMB)=((mP/2)t0/tP)/((mP/2).(t0-t)/tP)
z+1=t0/(t0-t)=t0/(t0-t)-(t0-t)/(t0-t)+1=t/(t0-t)+1
This is precisely
what I arrived from Starinets solution notes, so we have the same result using
a third (slightly different) approach.
---
In Solving the Hubble Tension by
Extracting Current CMB Temperature from the Union2 Supernova Database, Haug and Tatum get a third value for
z. This is established from use of his
(apparently) scaled temperatures. This
ends up with him comparing T0=TP/(4π.√(2ꬱ0))
with TCMB=TP/(4π.√(2ꬱCMB)) so, noting
that the subscript CMB here is equivalent to recomb used above:
z+1=TCMB/Tnow=Trecomb/Tnow=(TP/(4π.√(2ꬱ0)))/(TP/(4π.√(2ꬱrecomb)))
z+1=√(ꬱ0/ꬱrecomb)=√(t0/(t0-t))=√(36,000)
It should be noted
that they state that there was a choice between Tt=T0√(1+z)
and Tt=T0(1+z).
They chose the latter, but if we choose the former, we get:
z+1=(Trecomb)2/(Tnow)2=(TP/(4π.√(2ꬱ0)))2/(TP/(4π.√(2ꬱrecomb)))2
z+1=ꬱ0/ꬱrecomb=t0/(t0-t)=t0/(t0-t)-(t0-t)/(t0-t)+1
z=t/(t0-t)
So we have a fourth
approach to arrive at the same result (albeit via the questionable route of taking Haug and Tatum seriously).
---
Finally, returning
to Starinets’ solution notes, he notes that in the “matter-dominated era” a(t)=(t/t0)2/3. This is a Standard Model thing. In a FUGE universe, a(t)=(t/t0)
at all times, which is to say that there has been more stretching of the
universe over the period between recombination and today and more redshift, so
it’s precisely what we should expect.
---
As I have managed to find five* methods for arriving at equations for redshift which indicate that, for an event that occurred a period t ago, z=t/(t0-t), I am no longer convinced that there is a problem with my calculations.
There is however some
confusion on the part of Tatum et al. associated with the introduction of their
“composite constant” upsilon. This will be
touched on again in the next (shorter) post.
---
* I understand that not all five methods are entirely distinct. Also questions remain about Tatum's equations, not only with respect to his methods for getting the papers that contain them published but also with respect to whether the implied relationship he identifies via those equations is anything more than a rather startling coincidence.