How well do we know our Universe? (A Historical perspective on - - PowerPoint PPT Presentation

How well do we know our Universe?

(A Historical perspective on modern cosmology)

How well do we know our Universe?

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“Very well” The answer given by most human cultures at all epochs!

Some ancient ideas

• The ancient Indian concept
• f 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 20th 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

• riginated 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

I I Local Local data data II II Galaxies Galaxies III III Quasars Quasars IV IV ? ? Surface of last scattering Surface of last scattering V V Universe optically thick Universe optically thick Z = Z = ∞ Z = 1000 Z = 1000 Z = Z = 5.0 Z = Z = 1.0 Z = Z = 0.1 Z = Z = 0 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
• Tsecond = 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 1016
• GeV. Quantum gravity effects occur at energies
• f around 1019 Gev.

Astroparticle physics = 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. Very early universe Very high energy particle physics +

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? 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)

• ne is apparently able to confirm two speculations, namely

Yes! In astroparticle physics…

• 1. A grand unified theory operated in the very early epochs.
• 2. Inflation took place in the very early epochs.

FRW FRW Ω ΩΛ

Λ

N-Body Simulations N-Body Simulations Biasing Biasing Inflation Inflation Quantum Fluctuations Quantum Fluctuations DM DM The epicyclic universe:

(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…

Alternatives available:

• 1. since these contradictions exist we need to revise our

standard big bang model…

• 2. So, we believe the model to be true and are willing to

modify our view of what the dark matter is made of, even if it has no other direct observational support. As yet there is no direct astronomical or laboratory evidence for non-baryonic dark matter; yet it is asserted that it exists in far greater quantity than the normal baryonic matter. If dark matter turned out to be wholly baryonic, it will pose problems for the big bang, e.g., too little primordial deuterium, large scale structure would create too large fluctuations today in the radiation background.

Emperor's New Clothes?

Emperor's New Clothes?

(2) Epicycle of cold dark matter (CDM) What type of non-baryonic dark matter? If we want the current ideas on large scale structure to produce the right kind of structures and the observed fluctuations of the CMBR, then the non-baryonic matter must be cold…. No example of CDM is yet known to laboratory physics.

Emperor's New Clothes?

(3) The epicycle of cosmological constant Recall that earlier the big bang theorists discouraged the use of λ and had concluded that the universe is decelerating. Observations of distant supernovae cannot be explained unless the universe is accelerating. So the cosmological constant was recalled and accorded the place of honour.

However,…it soon became clear that this simple addition made by Einstein was not enough: it is necessary to have a variable λ. This feature is ‘explained’ by saying that there is ‘dark energy’ in the universe: No evidence otherwise exists for this hypothesis.

The subject needs an entirely different approach in which cosmology is seen as a natural extension of extragalactic astronomy. The modern picture seems based on a large number of speculative assumptions none of which is independently tested, whether as experimentally tested physics or

• bservationally tested astronomy.

“…I think it is very unlikely that a creature evolving on this planet, the human being, is likely to possess a brain that is fully capable

• f

understanding physics in its totality. I think this is inherently improbable in the first place, but, even if it should be so, it is surely wildly improbable that this situation should just have been reached in the year 1970.” Fred Hoyle (1970):

[1970 was prior to inflation, GUT, NBDM, Λ, CMBR anisotropies…]

A plea to the cosmologists: Please treat the subject as open with some room for alternative ideas…

Cosmologists today are like a flock of geese…