Flamsteed Astronomy Society

Successes and Problems of Big Bang Cosmology

— Prof Michael Joyce, September 23, 2005

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“...or are they parameters for the failure of our understanding?”

Distances  in Cosmology are expressed in ‘parsecs’.   One parsec is 3.3 light-years (roughly the distance to a nearby star).   The size (radius) of our Galaxy is about 15,000 parsecs.   The size of the Universe is about 10 thousand million (10,000,000,000) parsecs.

The notion of an expanding Universe leads to the hypothesis that at some point in the past the Universe was compressed in a point of zero size (a ‘singularity’ so-called because the equations break-down there), it started to expand (the Big Bang), and has been expanding ever since.

Today’s Big Bang Cosmology now provides enough framework for predictions to test development of the theory.  Interpretation of the data leads us to conclude that the time since the Big Bang (‘cosmic’ time) has been 10 to 12 billion years.   This gives a working definition for the ‘size’ of the Universe at 10 to 12 billion light-years, being the distance light can have travelled since the Big Bang.  Assuming light has always travelled at its present speed, we cannot observe parts of the Universe more distant that this even if they exist.  There hasn’t been enough time for light to reach us from there.  Development or elaboration of the theory, is concerned with —

The history of expansion.  How fast and when?

The form and state of the mass/energy in the Universe, and

The shape of the geometry of space-time.  Is it: —


positive curvature (like the surface of a sphere or cylinder); or

negative curvature (like the surface of a saddle).



Professor Michael Joyce is a theoretical physicist working at the University of Paris on the physics of large-scale structures (LSS) in galaxy clusters, researching the possible origins of these structures.   Michael was educated at Trinity Dublin and Princeton and has a background in particle physics.

He talked to us about the Successes and Problems of Big Bang Cosmology.  Cosmology, broadly put, seeks to determine a physical theory of the Universe.   This begs the definition of ‘Universe’ — generally the largest scale we can probe. Cosmology is a 20th Century science.   In 1915 the scope of the Universe was effectively our own Galaxy; many astronomers believed all nebulae were objects inside our Galaxy.   Cosmology presumes the principle that the underlying physics is the same everywhere in the Universe.  This may be impossible to prove however — astronomy is a science in which we can ‘see but not touch’.

BBC or Big Bang Cosmology has enjoyed remarkable success in explaining the apparent expansion of the universe and in particular in predicting the existence and nature of the cosmic microwave background radiation (CMB) and the abundances of the elements formed during nucleosynthesis in the Big Bang itself.

Recent observations however, have challenged the theory.  To explain the observed large-scale structures (LSS — clustering of galaxies), and the apparent acceleration in the expansion of the universe, cosmologists have had to invent the concepts of dark matter and dark energy, entities so far completely undetected despite intensive laboratory searches.  We are unsure if these notions are helpful or are just ‘parameters for the failure of our understanding’ much like the luminiferous aether of the 19th Century.

The present so-called standard model (called ‘FRW’ Friedman-Robinson-Walker, plus or minus Lemaitre) is founded on the notion of the Big Bang which developed from Einstein’s profound insights in his 1915 General Theory of Relativity.  Einstein’s equations described a universe where there was no absolute space or time, but where it was necessary to think about a space-time continuum which must be expanding or contracting.

Einstein fudged the equations to produce a static universe (a move he later called his biggest mistake), but by 1929 the brilliant observational work of Edwin Hubble (assisted by Milt Humason) had measured the red-shifts in the light spectra from several galaxies — the Universe appears to be expanding.   Red-shift is taken to be a measure of recessional velocity relative to us.  Hubble’s Law states that galactic red-shifts appear directly proportional to distance from us.  Distant galaxies seem to be receding proportionately faster than those nearby.   But the ’law’ is approximate and empirical (a relationship that fits the data but is not derived by prediction from underlying theory).

(picture Mike Dryland)