In the early 20th Century observations showed that what they thought were nebulae,
which would later turn out to be galaxies, were all redshifted. These observations seem
to show a shift of the spectra through what was thought to be a Doppler effect.
From this Hubble formulated the Red-Shift distance law of galaxies.
The observed linear relationship between the distance and red-shift seemed to imply large
receding velocities of distant galaxies and an enormous amount of kinetic energy.
They also indicated that all matter originated from a dense, hot state several billion years ago.
Franz Zwicky was very aware of the extraordinary implications of this
and suggested another mechanism to explain the observed red-shift.
He felt that photons lose energy as they propagate through space.
At the same time, Friedman and Lemaitre used Einstein's equations of general
relativity to come up with a solution in which space itself was expanding.
Their solutions became accepted as the mechanism producing the observed
red-shifts and are now part of the favoured hypothesis of the formation of the universe.
The cosmology implied by general relativity was not accepted by Hubble. In a series of lectures,
he explained that the familiar Doppler interpretation of red-shifts seems to
imply a strange and dubious universe, very young and very small. More recent
measurements estimate the universe to be 10 times larger and older than Hubble's
universe and the cosmological red-shift is explained by the expansion of space.
The mechanisms behind many red-shift observations remain unclear. The expansion of space does not
explain the solar limb red-shift, the K-effect, compact groups of galaxies with one discordant
red-shift member, intrinsic red-shift of quasars, quantised red-shifts, alignment
of galaxies showing the 'Finger of God' and the high degree of uniformity of the CMB radiation.
As a result, many models have been proposed. Some are based on ideas similar to Zwicky's tired
light model while others are based on Doppler red-shift, space expansion or even New physics.
We can roughly group the mechanisms as follows:
Those caused by effects which directly relate to their distance or some metric of space itself
Those caused by the geometry of space time
Those where a property of matter creates a red-shift
Those where the interaction between light and matter causes a red-shift photons
Those where some property of light causes a change over time
And finally where a field of some type interacts with the light causing a shift.
For each, I will give an overview of some of the alternative ideas. I will then also expand
on some of the more interesting concepts to discuss them in more detail. At this point,
it is fair to say that no single model is complete all have problems and it is likely that no single
model can account for all the red-shifts we see but that a combination of mechanisms is required.
In this first video, we will start by exploring the Time-dependent
distance or metric of space mechanisms. So without further ado let's dive in....
The Doppler Effect
This effect is named after Christian Andreas Doppler, who offered the first known physical
explanation for the phenomena in 1842. This effect is something we encounter
on a day-to-day basis most often with sound waves. The passing of a
car creates a marked shift in the pitch as it speeds towards and then away from you.
Doppler predicted that this effect should apply to all waves. This prediction turns
out to be correct. In 1848 French physicist Armand-Hippolyte-Louis Fizeau was the first
to point out that the shift in spectral lines seen in stars was due to a Doppler Effect.
In 1868 Willian Huggins would be the first scientist to use
this to determine the velocity of a star moving away from the Earth.
3 Years later optical redshift was observed in solar rotation. In 1901
Aristarkh Belopolsky would confirm this effect in a laboratory using a system of rotating mirrors.
This method is used to determine the radial velocity of nearby stars thereby allowing us
to map out the motions of these stars not just across the sky but towards or away from us.
Expansion of the Universe The initial interpretations
of the redshift and blueshifts of interstellar objects outside of our
Milky Way were based exclusively on the idea of the shift being due to the Doppler effect.
After Hubble discovered the correlation between the increasing redshift and the increasing
distances to galaxies. Friedman and Lemaitre soon realised that an alternative explanation could be
found using Einstein's general relativity. It was not the objects that were moving away
from us but instead the space in between which was expanding. This idea is different from the
Doppler effect in that the photons increase in wavelength and redshift as the space through
which they are travelling expands rather than the velocity of the source relative to the observer.
This type of redshift is often referred to as Cosmological redshift.
One important distinction between this and Doppler is that in the Doppler effect the object moving
cannot exceed the speed of light as this would be in violation of the laws of physics. This
is not the case in cosmological redshift, the space IS allowed to expand at any rate.
No experiment has ever been performed that verifies that expanding space can
indeed alter the wavelength of an already moving photon.
Einstein's comment to Lemaitre's was:
Your calculations are correct but your physics is abominable. The de Sitter universe based on
imaginary fabrication of a repulsive force varying directly with distance.
Lamba-Cold-Dark-Matter Cosmology
Lambda is the cosmological constant which was introduced
by Einstein to counter balance the effect of gravity and achieve a static universe.
It was later removed and much later revived and reinterpreted as the energy density of
space and is closely associated with the concept of Dark Energy.
During the 1970s and 1980s various observations showed that there was
insufficient visible matter in the universe to account for the
apparent strength of gravitational forces within and between galaxies.
In order to make up for these observations, Big Bang theorist whimsically invented dark matter.
This supposedly makes up around 90% of the matter in the universe, does not emit light nor interact
with normal matter. The evidence they claimed for the existence of dark matter was that the
universe was far more lumpy, the rotation curve of galaxies do not fit a model with ordinary matter,
and the motion of galaxies within galaxy clusters implied additional mass must be hiding somewhere.
Dark matter does not appear to interact with normal matter or
radiation in anyway except for gravitationally.
To date the vast amounts of money spent on trying to detect Dark matter have all failed.
On top of this measurements of red-shift of type 1a supernovae indicate that if the assumptions
that the Big Bang theory relies on are true, then the universal expansion is actually accelerating.
To explain this general relativity requires that a lot of the energy of
the universe consists of a large negative pressure. This is called dark energy
Here is a chart comparing a universe where only the Doppler Effect is the cause, one where we
have an expansion of space and lastly where we have Dark Energy Accelerating the Universe.
I hope you have enjoyed the video in the next part we will continue the journey looking at
other Redshift Mechanisms. I would just like to give a big shout-out to all my Patreon supporters
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