Time dilation is a fundamental property of an expanding universe.
In an expanding universe, it is directly linked to cosmological redshift.
This means that events observed at cosmological distances, so with a significant redshift,
appear to take longer than they would if they were nearby.
As space expands the light takes increasing amounts of time to reach us over the course
of the event.
The lengthening ratio is the same ratio as the redshifted wavelengths from the same distance.
There have been a number of claims by groups working on gamma-ray bursters that time dilation
is seen in the stretching of peak-to-peak timescales.
This has then been used to support the argument that the bursts are at cosmological distances.
It is questionable if the argument can be inverted to provide convincing evidence for
the existence of time dilation.
More direct observations of time dilation have come from the measurements of the decay
time of distant supernova light curves and spectra.
Although here I would point to the video I did discussing the fact that there is a growing
catalogue of supernova which simply do not follow the rules on timings and behaviour.
So the idea that these can be used as a time measurement could be called into question.
Nonetheless, their results would seem to provide strong evidence that time dilation has been
observed.
If time dilation occurs in supernova it must also happen in some of the oldest objects
we can observe, quasars.
A variety of groups have looked for time dilation in quasar light curves but so far no convincing
evidence has been found.
In this episode, I want to explore how the data from one paper is interpreted in two
different ways where the authors of both papers come to very different conclusions both of
which contain important considerations.
Let's dive in and find out more.
We will start by examining the paper by M. Hawkins titled 'Time dilation and Quasar variability'
and then examine a paper by C. Rourke who uses Hawkins' data to come to a very different
conclusion.
In order to measure time dilation in quasar light curves it is necessary to find a way
of characterising the timescale of variation.
The most popular parameterisation to date has been the structure-function, and several
groups have measured it for samples of light curves.
The main drawback of this is that the points are independent of each other, which causes
error analysis and makes it very difficult to interpret.
Fourier power spectrum analysis has not been used much in the analysis of quasar light
curves.
It requires a long run of evenly spaced data but provides some significant advantages over
the other methods which are primarily in the interpretation of the data.
Hawkins uses this method to analyse a large sample of quasars which have been monitored
every year for 24 years.
In total, they identified 600 quasars with redshifts in the range of 0.1 to 3.5.
All the quasars used in the study fluctuated significantly in brightness over the 24-year
period.
For the study of time dilation, they first made the assumption that the light variations
are intrinsic to the quasar, and so the light curves are subject to the effects of time
dilation.
They then rescaled the time interval to effectively remove the effects of time dilation.
This should result in no trend of timescale with redshift.
In this graph, we can see the power spectra of samples of quasar light curves, binned
according to redshift and luminosity.
Each point represents the average value of all the contributions to that frequency interval.
The top two panels show results for two luminosity bins, with power spectra for low and high
redshift quasars being represented by filled and open circles.
If the timescales of the quasar light curves were subject to time dilation one would expect
no displacement between the two curves.
The two power seem to show clear displacement.
The power spectra are separated by about 0.15 in the log in both luminosity bins.
This is close to the offset produced by allowing for a (1+z) scale change, on the basis of
the mean redshifts of the bins.
There is also an indication that the power spectra have moved horizontally rather than
vertically.
The bottom two panels show similar data for two redshift bins.
In this case low and high luminosity quasars are represented by closed and open circles
respectively.
The mean redshift for the high and low luminosity data in each plot only differs by about 0.03
in the log, and therefore the removal of (1+z) factor should make little difference.
The two luminosity bins are well separated which implies more power or shorter timescales
for low-luminosity objects.
Hawkins makes no assumptions about the nature of the quasar variability in the paper but
carried out the power spectrum analysis in the observer's reference frame.
In this diagram we see the power spectra of quasar light curves as before but this time
with no correction for time dilation.
For the two luminosity bins in the top two panels, it will be seen that all spectra show
a well-defined linear relationship, in other words, the power law.
On the basis of the mean redshift for each of the bins, the effects of time dilation
should result in a horizontal offset of 0.15 between the power spectra in each of the two
top panels.
In fact, in each case, the high and low redshift data are superimposed, showing no change of
timescale with redshift.
The two redshift bins in the bottom two panels once more show well-defined power laws for
the power spectra, and in this case, it is clear that low luminosity quasars have more
power on shorter timescales.
This effect can also be seen by comparing the slopes of the power spectra for low and
high luminosity quasars in the top two panels and confirms that the power spectra are consistently
measuring changes in timescale.
So what does this all mean?
Hawkins uses this data to point out that this implies quasars do not suffer the effects
of time dilation.
This is of course a big problem for the idea of an expanding universe where there must
be time dilation.
Hawkins offers a number of alternatives to explain the discrepancy.
If the timescale of the quasar variation were a function of wavelength in the sense that
timescales were shorter in bluer passbands then this might possibly exactly offset the
effect of time dilation.
This could be directly tested.
In this diagram, we see the power spectra for a sample of about 200 quasar light curves
in blue and red passbands.
If there were a correlation of timescales with wavelength then the two power spectra
should be separated by 0.18 on the log scale, corresponding to the effective wavelength
of 436nm and 665nm for the blue and red passband.
In fact, the data for the blue light curves appear to be systematically offset from the
red by 0.06 in the log.
Another alternative he suggests is that the timescale of quasar variation decreases by
a factor of (1+z) towards high redshift by some as yet unspecified physical process,
to exactly cancel out the time dilation effect.
This would require an almost cosmic conspiracy to achieve the required fine-tuning and the
plots shown would make this almost impossible.
So how can we explain the lack of time dilation seen in quasars?
Hawkins outlines 3 possible reasons.
All of which conflict with the broad consensus in the astronomical community.
The first is that time dilation is not a property of the universe.
The reason mainstream astronomy requires time dilation is because the universe is expanding.
So no expansion would also mean no time dilation.
The second possibility is that quasars are not at cosmological distances.
Although hotly disputed back in the 1970s there is now evidence that shows quasar host
galaxies at high redshifts making this an unlikely possibility in his view.
The third possibility he outlines is that the observed variations are not intrinsic
to the quasars but caused by some intervening process at lower redshifts, such as gravitational
microlensing.
There is an opposing view that the variations in quasars are dominated by instabilities
in the accretion disc but the debate centres on whether this mechanism is responsible for
the long timescale large amplitude variations which dominate the power spectra discussed
in this paper.
He concludes his paper by admitting that none of these explanations are satisfactory and
leaves open the question of the absence of a time dilation effect in quasar power spectra.
Let's now move on to a response paper by C. Rourke based on Hawkins' original paper.
He starts by re-iterating the fact that redshift and time dilation are effectively identical
in general relativity and then stresses the paradoxical findings from Hawkins' paper where
we see redshift without time dilation.
He moves on to discuss Hawkins' results and points out that the results need not be paradoxical
and that instead, it shows that for the selection of quasars the sources of radiation and the
time variation are not in the same place.
For the ease of discussion, he calls these the generator and the modulator.
Although mainstream has a clear definition of what the generator is there is no similar
consensus for the modulator.
For active galaxies, it is believed that there is a central black hole and that this is surrounded
by a rotating accretion disc or other structure.
Quasars and active galaxies are considered to be part of a spectrum of phenomena associated
with black holes.
Therefore quasars should also be surrounded by similar rotating accretion structures.
These are composed of gas and plasma close to the centre and further out more solid objects
all of which are trapped in orbit around the central mass.
The radiation from the generator passes through these surrounding layers on its way to us.
He goes on to point out that therefore the observed variations are due to non-uniformity
in these layers, and are naturally periodic with the possibility of several different
periods coming from different layers superimposed.
So how would this explain Hawkins' results?
Rourke's idea is that generator is subject to gravitational redshift because of the black
hole nearby and because the modulator is some distance away it is not subject to the same
redshift and this explains the dichotomy between Hawkins' results.
Because of the gravitational redshift, the quasar is close and less luminous than would
be the case if redshift were entirely cosmological.
This is therefore an intrinsic redshift and implies that for a large class of quasars,
the generator and the modulator are some distance apart with the modulator in a region of low
cosmological redshift.
The modulator must lie on the light path from the generator to us.
There are two possibilities he outlines.
It lies close to the generator, which therefore has intrinsic redshift, or, it lies on the
path to use and close enough to us for the cosmological redshift to be small.
He rules out this last possibility as implausible leaving just intrinsic redshift as his solution.
At this point it is probably useful to dive a little deeper into his model.
In a separate paper, he wrote together with Enriquez and MacKay they explain it in more
detail.
One of the reasons gravitational redshift has been disregarded for quasars concerns
angular momentum.
If the surrounding medium of the black hole has even a very small angular momentum about
the centre, then conservation of angular momentum will create large tangential velocities as
infalling matter approaches the centre and this will tend to choke off the inflow and
prevent accretion.
This has led to the subject being dominated by the theory of accretion disks.
If the observed radiation comes from an accretion disk affected by local gravitational effects,
there would be wide spectral lines, redshift gradient, and not narrow ones as observed.
A simple consequence of the inertial drag effect is that a rotating body can absorb
angular momentum.
If the angular momentum can be nullified by central rotation, then it does not force the
existence of an accretion disk and redshift can be largely gravitational.
In their model, black holes radiate by converting the gravitational energy of incoming matter
into radiation.
There are two significant regions: an optically thin outer region and an optically thick inner
region which are separated by a sphere which we call the Eddington sphere.
The radiation that we see comes from a narrow band near the Eddington sphere and which is
all at roughly the same distance from the central black hole.
This allows the radiation to exhibit a consistent redshift and no redshift gradient would be
present.
They go into a lot of detail outlining the model but to summarise quasars are not distant
objects but are instead objects which are much closer.
They have a black hole at their centre, they do not have an accretion disk and the redshift
we observe is intrinsic and is created via gravitational redshift.
Now one point to consider here is one we discussed when looking at the Variable Mass theory.
The Lyman alpha forest problem.
In this paper, they actually use this as a piece of evidence.
To recap these are thought to be produced by clouds on the path to us which are at different
redshifts so create a series of absorption lines that change.
They outline that the forest suggests strongly that there is a settling process that takes
place in the outer region, forming strata.
These strata are responsible for both the observed Lyman alpha forest but also the quasar
variability.
They do concede that there are complicated features for many quasars which are not adequately
explained by this simple model even with added absorption clouds and strata.
For heavier quasars, especially those with jets this model is not sufficient.
I think the most important take away from both of these papers is:
- Based on Hawkins' analysis there is clearly a large set of quasars that do not exhibit
any time dilation.
This cannot be explained using the current model of quasars nor an expanding universe
- I'm intrigued by Rourke's paper as it presents a very different concept for looking at this
problem.
By separating the generator from the modulator it is possible to look at the problem in a
new light.
The idea of there being a series of strata surrounding the central object and that it
is the settling of these that creates the Lyman alpha forest is a very interesting concept
that bears consideration for other theories that rely on intrinsic redshift.
I would be interested to know if anyone else has come across either of these two papers
and in particular Colin Rourke's work.
He seems to have written and co-written a number of papers which cover a concept without
a Big Bang, dark matter, dark energy or an expanding universe.
This may not have been that obvious from the bits I included here of his work but may be
worth exploring in more detail.
I will put a link to his book on a new paradigm for the universe as well as the other links
to the papers I discussed down below in the description.
I hope you have found this a useful insight in a slightly different direction when considering
the origin of redshift.
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