If space-time expanded together with matter then why do physicist bother extrapolating backwards the expansion back to a point in time? I mean does that really tell us anything? I mean if the speed of the passage of time (measured by atomic vibrations) is dependant on the local space-time geometry, and the measurement of space is dependant on having 'stuff' (light, matter) in it to measure the distances, then it seems to me that the concept of space, time, its interacting fields, and the resulting quantum actualities are all dependant on each other for their survival as a concept.

What explanatory power have we really added to the discussion on the origins of reality if we talk about a finite time periods back to the infinite? How can a finite time period escape from the infinite? Therefore what are we really learning by saying the universe is 14 billion years old? In my ignorance, its like we are applying logic based on newtonian physics to post Einstein physics and walking away happy that we said something sensible when we just got ourselves (and me especially ;) really confused (I'm sure the reality is different, thats why I'm asking the question :)).

Furthermore, if everything (within the fabric of nothing) expanded together then how could we prove anything expanded at all? The red-shift must be showing that matter expanded FASTER than the space-time right? Otherwise there would be no measurable difference. How could we ever measure that if space-time is expanding with it?

I guess what I'm asking is 3 questions (that are really all the same question); why do we talk about space-time expanding when "expansion" is a term that needs space to make sense. Wouldn't the permission to use that term require our space-time existing in a greater spacial reality?

Why do we talk about matter expanding when if the space-time expanded with it then it really didn't expand at all.

Why do we talk about a finite time period when the maths shows the passage of time retreats to infinity and therefore we haven't really explained anything.

  • $\begingroup$ Why am I getting down votes? I desperately want to understand the big bang!! $\endgroup$ – Mike S May 17 '14 at 10:33
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    $\begingroup$ Not my downvote, but the expansion doesn't affect matter on the small scale. See Why does space expansion not expand matter? for a discussion of this. For more on the Big Bang search this site for flrw metric and you'll find lots of related questions. $\endgroup$ – John Rennie May 17 '14 at 10:50
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    $\begingroup$ Although there are no exact duplicates several existing questions address the issue of whether the spacetime expansion affects matter $\endgroup$ – John Rennie May 17 '14 at 10:53
  • $\begingroup$ Does this answer one of your questions? physics.stackexchange.com/q/104153/29216 $\endgroup$ – BMS May 17 '14 at 16:08

We started from observations: galaxies and clusters of galaxies have been found to be retreating from our galaxy and each other.

Hubble's law is the name for the observation in physical cosmology that: (1) objects observed in deep space (extragalactic space, ~10 megaparsecs or more) are found to have a Doppler shift interpretable as relative velocity away from the Earth; and (2) that this Doppler-shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.1 This is normally interpreted as a direct, physical observation of the expansion of the spatial volume of the observable universe.

Once we model the space around us expanding, the way a loaf of bread expands when rising in the oven, then the question of matter also expanding is answered by the same observation: if it were, we would not be able to detect the expansion because our units would be changing in tandem.

Since we do see expansion, lets stretch the bread analogue a bit further> Let us put half walnuts in the bread. What will happen is the walnuts will be expanding with the bread, but their size will not change. Why? Because they are solid, held together with molecular forces and while the dough is elastic and expands distancing one walnut from the other the intermolecular forces keep the walnut intact.

In a similar way the collective gravitational Newtonian force which holds for local areas of the universe is much stronger than the current expansion of space, and the electromagnetic and strong forces are even stronger. Thus matter holds together. Radiation emitted and traveling through expanding space is affected and that is why we can deduce the velocities of galaxies and clusters receding from us.

It was not always so . We have made a pictorial representation of this expansion with the model of the Big Bang:

history of universe

We have had for some time data of the Cosmic Microwave Background radiation (CMB), a snapshot of what happened 380.000 years from the singularity. Recently the BICEP2 experiment claims to give us a snapshot of what happened at 10^-32 seconds from the singularity, the effect of quantization of gravity. Our model of the history of the colored region, before the CMB was released, depends on our knowledge of interactions from particle physics and the standard model that fits the particle data very well, and also data from nuclear physics.

This is a model that blends what we know of theory and experiment and observational astronomy, it is the state of the art at the moment.

Is there really a singularity at the beginning of time? We do not know. The general relativity solution says so, but it is not a quantized solution. Quantization is necessary during the inflationary period in the beginning, to explain the uniformity of distribution of matter in the observed universe. Most physicists expect that once quantization of general relativity is formulated correctly singularities may disappear, but we do not know. At the moment it looks as if there existed a singularity at the beginning of space time, one can fit all observations with such a model.

  • $\begingroup$ You rephrased what I was trying to say a lot more eloquently in "we would not be able to detect the expansion because our units would be changing in tandem", but I didn't find an explanation of how we can ever detect that space is in fact expanding. $\endgroup$ – Mike S May 17 '14 at 13:42
  • $\begingroup$ The acceleration between distant objects can only be measured by it's redshift and the speed of light is relative to the geometry of the space-time it travels through. Therefore, if our measuring stick does grow as space grows, it makes detection of said space impossible? How do we know that the matter didn't explode in the middle of an already spacious universe bubble? Would that be identical to the observed red-shifts effect? $\endgroup$ – Mike S May 17 '14 at 13:42
  • $\begingroup$ Our measuring stick is here on this earth and now, and it does not grow because it is made of matter as my argument says. I do not understand your second question. The observable universe can be consistently, i.e. without contradictions, model by the big bang. Physics chooses the most economical hypothesis that fits the data. $\endgroup$ – anna v May 17 '14 at 15:03
  • $\begingroup$ Oh ok. So we use space, as it is today, to compare to the ancient space we can see (through the telescopes). $\endgroup$ – Mike S May 19 '14 at 1:13

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