The guest speaker at the October monthly meeting of Keighley Astronomical Society was our own society member Dr Adrian Smith. This was the first such presentation he had delivered and the first time he had lectured to an audience on this subject matter. It was a clear and understandable presentation on a topic that is enigmatic to say the least. The ‘A to Z of Cosmology’.
The term Cosmology comes from Ancient Greek κόσμος (kósmos) ‘world’, and -λογία (-logía) ‘study of’) and is a branch of physics and metaphysics dealing with the nature of the universe. The term cosmology was first used in English in 1656 in Thomas Blount’s publication ‘Glossographia’, and in 1731 taken up in Latin by German philosopher Christian Wolff, in ‘Cosmologia Generali’.
So what is Cosmology?
It is the study of the observable universe’s origin, its large-scale structures and dynamics, and the ultimate fate of the universe, including the laws of science that govern these areas. It is investigated by scientists, such as astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions, and may depend upon assumptions that cannot be tested. Modern physical cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics; more specifically, a standard parameterization of the Big Bang with dark matter and dark energy, known as the Lambda-CDM model.
Theoretical astrophysicist David N. Spergel has described cosmology as a “historical science” because “when we look out in space, we look back in time” due to the finite nature of the speed of light.
Adrian explained that modern scientific cosmology is usually considered to have begun in 1917 with Albert Einstein’s publication of his final modification of general relativity in the paper “Cosmological Considerations of the General Theory of Relativity” (although this paper was not widely available outside of Germany until the end of World War I). Prior to this the universe was basically what could be seen in the observable night sky. The initial problem was the difficulty in measuring distances. In 1917 Harlow Shapley as only able to measure the Milky Way as being 100,000 light years across.
General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild, and Arthur Eddington to explore its astronomical ramifications, which enhanced the ability of astronomers to study very distant objects. Physicists began changing the assumption that the Universe was static and unchanging. In 1922 Alexander Friedmann introduced the idea of an expanding universe that contained moving matter.
In parallel to this dynamic approach to cosmology, one long-standing debate about the structure of the cosmos was coming to a climax ‘The Great Debate (1917 to 1922)’ with early cosmologists such as Heber Curtis and Ernst Öpik determining that some nebulae seen in telescopes were separate galaxies far distant from our own. While Heber Curtis argued for the idea that spiral nebulae were star systems in their own right as island universes, Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only. This difference of ideas came to a climax with the organization of the Great Debate on 26th April 1920 at the meeting of the U.S. National Academy of Sciences in Washington, D.C.
The debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda Galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way. Hubble began to catalogue other distant objects, which were recognised as other distant galaxies, and found that they were at even greater distances. Further more by measuring their Doppler shift he realised there was a linear relationship between distance and the speed of recession. This is what we know as ‘Hubble’s Law’
Subsequent modelling of the universe explored the possibility that the cosmological constant, introduced by Einstein in his 1917 paper, may result in an expanding universe, depending on its value. Thus the Big Bang model was proposed by the Belgian priest Georges Lemaître in 1927 which was subsequently corroborated by Edwin Hubble’s discovery of the redshift in 1929 and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964. These findings were a first step to rule out some of many alternative cosmologies.
The term ‘Big Bang’ was coined by Fred Hoyle in a radio broadcast in 1952 as a derogatory epitaph. Hoyle who was born and raised in Bingley was a form believer in the steady state theory. He became the de facto leader of the ‘Steady state theory’.
George Gamow (A Ukrainian born American scientist) suggested in the 1940’s that if you could look deep enough into space you should see the radiation left over from the ‘Big Bang’. He even suggested that the Bell laboratory’s in New Jersey would be ideal to search for this radiation. Forward to 1965 and two researchers Arno Penzias and Robert Wilson acquired the use of the Bell laboratories antenna at Homdel. They wanted to do some radio astronomy, but all they got was hiss.
For over a year they tried in vain to track down what they thought was a fault producing this constant hiss. In desperation Penzias and Wilson contacted Robert Dicke at Princeton University who suggested it might be the background radiation predicted by some cosmological theories. The pair agreed with Dicke to publish side-by-side letters in the Astrophysical Journal, with Penzias and Wilson describing their observations and Dicke suggesting the interpretation as the cosmic microwave background radiation (CMB), the radio remnant of the ‘Big bang’. This allowed astronomers to confirm the Big Bang, and to correct many of their previous assumptions about it.
Dark Matter – What is the evidence?
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called “dark” because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect. Various astrophysical observations; including gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen, imply dark matter’s presence. For this reason, most experts think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution.
The primary evidence for dark matter comes from calculations showing that many galaxies would behave quite differently if they did not contain a large amount of unseen matter. Some galaxies would not have formed at all and others would not move as they currently do.
Other lines of evidence include observations in gravitational lensing and the cosmic microwave background, along with astronomical observations of the observable universe’s current structure, the formation and evolution of galaxies, mass location during galactic collisions and the motion of galaxies within galaxy clusters.
In the standard Lambda-CDM model of cosmology, the total mass-energy content of the universe contains 5% ordinary matter and energy, 27% dark matter, and 68% of a form of energy known as dark energy. Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass-energy content.
Because no one has directly observed dark matter yet (assuming it exists) it must barely interact with ordinary baryonic matter and radiation except through gravity. Dark matter is thought to be non-baryonic; it may be composed of some as-yet-undiscovered subatomic particles. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, particularly weakly interacting massive particales (WIMPs).
Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded. Dark matter is classified as “cold,” “warm,” or “hot” according to its velocity (more precisely, its free streaming length). Current models favour a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles.
Although the scientific community generally accepts dark matter’s existence, some astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter.
The Bullet Cluster (1E 0657-56) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 3.72 billion light-years.
The major components of the cluster pair; Stars and gas, and the putative dark matter, behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the baryonic, or “ordinary”, matter in the cluster pair. The gases of the Intracluster medium interact electromagnetically, causing the gases of both clusters to slow much more than the stars.
The third component, the dark matter, was detected indirectly by the gravitational lensing of background objects. In theories without dark matter, such as Modified Newtonian Dynamics (MOND), the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near (possibly coincident with) the visible galaxies. This provides support for the idea that most of the gravitation in the cluster pair is in the form of two regions of dark matter, which bypassed the gas regions during the collision. This accords with predictions of dark matter as only gravitationally interacting, other than weakly interacting.
The Bullet Cluster provides the best current evidence for the nature of dark matter and provides “evidence against some of the more popular versions of Modified Newtonian dynamics (MOND)” as applied to large galactic clusters.
Particularly compelling results were inferred from the Chandra observations of the ‘bullet cluster’ (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4,500 km/s) merger, evident from the spatial distribution of the hot, X-ray-emitting gas, but this gas lags behind the sub cluster galaxies. Furthermore, the dark matter clump, revealed by the weak lensing map, is coincident with the collisionless galaxies, but lies ahead of the collisional gas. This and other similar observations allow good limits on the cross-section of the self-interaction of dark matter.
According to Eric Hayashi:
The velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured Lambda-CDM model cosmology.”
A 2010 study claimed that the velocities of the collision as currently measured are “incompatible with the prediction of a LCDM model”. However, subsequent work has found the collision to be consistent with LCDM simulations, with the previous discrepancy stemming from small simulations and the methodology of identifying pairs. Earlier work claiming the Bullet Cluster was inconsistent with standard cosmology was based on an erroneous estimate of the in-fall velocity based on the speed of the shock in the X-ray-emitting gas.
Based on the analysis of the shock driven by the merger, it was recently argued that a lower merger velocity ~3,950 km/s is consistent with the Sunyaev–Zeldovich effect and X-ray data, provided that the equilibration of the electron and ion downstream temperatures is not instantaneous.
However Mordehai Milgrom, the original proposer of Modified Newtonian dynamics, has posted an online rebuttal of claims that the Bullet Cluster proves the existence of dark matter. He contends that the observed characteristics of the Bullet Cluster could just as well be caused by undetected standard matter.
Another study in 2006 cautions against “simple interpretations of the analysis of weak lensing in the bullet cluster”, leaving it open that even in the non-symmetrical case of the Bullet Cluster, MOND, or rather its relativistic version TeVeS (tensor–vector–scalar gravity), could account for the observed gravitational lensing.
In 2013, the European-led research team behind the Planck cosmology probe released the mission’s all-sky map of the cosmic microwave background. The map suggests the universe is slightly older than researchers expected. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about 370000 years old.
The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter. Based on the 2013 data, the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. On 5 February 2015, new data was released by the Planck mission, according to which the age of the universe is 13.799±0.021 billion years old and the Hubble constant was measured to be 67.74±0.46 (km/s)/Mpc.
Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project, Atacama Cosmology Telescope and the QUIET telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization.
The evidence for Dark Energy?
The evidence for dark energy is indirect but comes from three independent sources:
Distance measurements and there relation to redshift, which suggest the universe has expanded more in the latter half of its life.
The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature).
Measures of large-scale wave patterns of mass density in the universe.
Dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the universe does not expand at a constant rate; rather, the universe’s expansion is accelerating.
Understanding the universe’s evolution requires knowledge of its starting conditions and composition. Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background (CMB) suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how scientists could measure an accelerating universe.
Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research to understand the fundamental nature of dark energy. Assuming that the lambda-CDM model of cosmology is correct, as of 2013, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. Dark energy’s density is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe’s mass-energy content because it is uniform across space.
Adrian’s closing remarks were the standard model of cosmology has been the dominant paradigm for astronomy for the last thirty years. It can be broken down into:-
The ‘Big bang’ Theory
By far the best understood components are the ’Big bang’ theory and Nucleosynthesis. Inflation makes good predictions, but does not have any direct evidence in its favour. However there are no serious contenders.
There are now several serious competent alternatives to dark matter including dynamical theories that do not need any new physics. Even the dynamics of Galaxy clusters and computer simulations have shown to work without dark matter.
Dark energy is even more mysterious with very little progress delineating this. Recent works also cast doubt on its validity.
If the practitioners of the standard model of cosmology want their theory to be considered the equal of the standard model of partial physics; they need to believe the evidence, even if it is the null result. They need to be transparent in their methods, and have a clear boundary between conjecture and observation.