How well do we know our Universe? (A Historical perspective on modern cosmology)
How well do we know our Universe? “Very well” The answer given sj1.docx by most human cultures at all epochs !
Some ancient ideas • The ancient Indian concept of Brahmand …or : “The cosmic egg”
The Norse World Tree
The hierarchical universe
Symptoms of being on the wrong track: An idea that failed when facts could not support it: Pythagorean Counter-Earth…
Enter…Science All statements must be testable and accepted only if checked by Observations… The three A’s from Greece… Aristarchus, Aristotle, Archimedes
• Archimedes: important ideas in laboratory physics • Aristarchus: heliocentric theory • Aristotle: ideas on fundamental physics and the cosmos
The decreasing importance of Man on the Earth Copernicus repeated Aristarchus’s claim… Galileo’s ‘wrong’ defence of Copernicus… Eventual proof that the Earth moves came from observations of aberration and parallax *** Herschel’s picture of the Milky Way shows the Sun at the centre
Caution : Some of the cosmological facts known today became established despite majority opinion against it… 1. The Earth moves round the Sun 2. The Sun is not at the centre of galaxy.. 3. There are other galaxies like our own lying very far away…
• Alas! By the early 20 th century it was realized that the Sun is two thirds of the way towards the boundary of the Milky way…
Where is the Galactic Centre?
Is our Galaxy alone in the universe? • The majority view was that our Galaxy is the only one that we see. • Nebulae like these were all believed to be part of the Milky Way.
Are all these nebulae in the Galaxy? Are all these nebulae in the Galaxy?
Majority View as expressed by Agnes Clerke On Nebulae... ‘...The question whether nebulae are external galaxies hardly any longer needs discussion. It has been answered by the progress of research. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of co-ordinate rank with the Milky Way...’ ( The System of the Stars, 1905, p. 349).
The majority view turned out to be wrong! • With better observing techniques it became clear that… • Some nebulae are in the Milky way but the majority are outside it… • They are galaxies like the Milky way.
Cosmological Revolution • In 1917 Albert Einstein applied his general theory of relativity to model the universe… • He expected the universe to be static but could not get such a model. • So he revised his theory to include a new force of repulsion. This force was of the form distance × λ , where λ is a constant called the cosmological constant .
Hubble’s Law (1929) • Edwin Hubble found in 1929 his famous relation • v = H × D • The farther ( D ) a galaxy from us the faster ( v) it moves away. • (Abbe Lemaitre found this relation in 1927)
Standard Cosmology • 1922: A. Friedmann predicted the expanding universe • 1929: E. P. Hubble found the m-z relation.
Explanation of the expanding universe • The way theory anticipated observations in this case is a strong plus point of the whole picture. • Einstein was for dropping λ , as a static universe was no longer needed. Friedmann’s models with or without λ gave a wide range of models…
All models predicted that the universe originated in a “BIG BANG” .
Abbe Lemaitre called the starting point as a “Primeval Atom” Fred Hoyle termed it the “Big Bang”. [Although Hoyle never believed in this model, the name he gave has stuck! ]
What is Big Bang? • It is infinitely dense, infinitely hot state of the universe when it had no well-defined spacetime geometry. • Mathematicians would call it a singular epoch… • Physicists have been attempting to go as close to this epoch as possible. A beginning was made by George Gamow.
• 1946-48: George Gamow used the aspect of the early universe that it was very hot, to predict that all chemical elements were synthesized by the fusion of neutrons and protons in the first three minutes after the big bang. Because of its three authors Alpher, Bethe and Gamow, this came to be known as the α−β−γ theory.
• 1948: R.A. Alpher and R.C. Herman, based on the early hot universe hot universe picture of George Gamow et al, predicted that there should be a relic cool radiation background today. They speculated that its temperature will be ∼ 5K. • There had been predictions of such a background of around 3K by Eddington and others earlier. • Standard cosmology predicts a thermal background today but not its temperature.
• Evidence for such a background already existed, vide observations of A. McKellar (1941). • Penzias and Wilson (1965) rediscovered this background. In 1990 the COBE satellite accurately established its thermal spectrum with a temperature of 2.73K.
…Thus observed facts are seen to confirm a speculation about the early universe.
The last scattering surface, however, places a limit on how far or how early a universe we can observe. Present epoch Present epoch Z = 0 Z = I I Local Local data data Z = 0.1 Z = II II Galaxies Galaxies Z = 1.0 Z = III III Quasars Quasars Z = 5.0 Z = IV IV ? ? Surface of last scattering Surface of last scattering Z = 1000 Z = 1000 V V Universe optically thick Universe optically thick Z = ∞ Z = Big bang Big bang
The very early universe • The successes of primordial nucleo- synthesis and the MBR inspired cosmologists to venture closer to the Big Bang… starting a new subject: the Astroparticle physics. Extrapolation of physics to very high energies leads to a time-energy relationship as follows.
Time temperature/energy relationship • Einstein’s equations give • T second = 2.4 g -1/2 T - 2 MeV • Gamow’s work on nucleosynthesis was around the MeV range. Unification of weak and electromagnetic interaction happens around 80 Gev. Grand unification may be around 10 16 GeV. Quantum gravity effects occur at energies of around 10 19 Gev.
Astroparticle physics = Very high energy particle physics Very early universe + Particle physicist: Since the big bang is established and secure as a theory of cosmology let me try my speculations of very high energy particle physics in this background… Cosmologist: Since particle physicists know what they are talking about, let me apply their theories to test my speculations of the very early universe.
Astroparticle Physics • The very early universe is the poor man’s high energy accelerator LHC ~ Big Bang?
Inflation • One of the ideas from astroparticle physics popular among cosmologists is of ‘inflation’. This occurred at the GUT-epoch when the grand unified theory split into strong and electroweak components. This phase Transition is supposed to have caused a fast expansion (inflation) of the universe.
Theories and Speculations • The last scattering surface prevents us from directly observing the state of the universe at earlier epochs. [Analogy of stage behind the curtain] • If not directly observable, one can study those early epochs with a well-tested theory…[e.g. Gamow’s nucleosynthesis]
The status of inflation But we do not have any well tested theory within energies from 1000 GeV to 10 16 GeV. So ideas like ‘inflation’ represent speculations rather than well-founded scientific theories.
Can two speculations together add up to a fact? Yes! In astroparticle physics… Example of two speculations: If there are GUTs, then there would be magnetic monopoles with relic density abnormally high. If there is inflation also, then these monopoles are washed away…so from a null observation (no monopoles seen today) one is apparently able to confirm two speculations, namely 1. A grand unified theory operated in the very early epochs. 2. Inflation took place in the very early epochs.
The epicyclic universe: Quantum Fluctuations Quantum Fluctuations Inflation Inflation Biasing Biasing DM DM N-Body Simulations N-Body Simulations Ω Λ Λ Ω FRW FRW
(1) The epicycle of ‘Non-baryonic dark matter’ Astronomical evidence exists for dark matter that exceeds what is visible…but by how much? Simplest alternatives known to astrophysicists: black holes, brown dwarfs, jupiters, or other non-shining matter…but made of neutrons and protons as in the visible case. This is the baryonic option…
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