The telescoper has an interesting post about how the expansion of the universe should be interpreted properly. The “expansion of space” in the framework of the Big Bang theory has been well described by both theory and observation, but when it comes to interpreting and visualizing the expansion, many cosmologists get confused.

First, I like Peter Cole’s allusion to ‘metaphors’ in physics. It is very brave to describe that teaching physics involves “ever-decreasing circles of deception”, and that some concepts are “more meaningless than incorrect”. But I’d prefer to describe it differently and associate these ‘metaphors’ with the reductionist approach inherent in physics. Although physics is one of the most exact and rigorous sciences whose predictions are widely reproducible and quantifiable in precise formulae, one has to remember that the real world is more complicated than could be described by simplistic theories. The reductionist approach in physics provides a way to make sense of complex systems by probing their fundamental constituents, however there also exists inhomogeneity and friction between those constituents which have to be taken into account before applying purely idealistic theories to the physical reality.

It was Hubble who first formulated his observations of the expanding universe in terms of the redshift of receding galaxies. Since then, cosmology has gone through a complete restructuring where steady-state models were abandoned in favor of the Big Bang scenario. Our current understanding of the universe is that we live in an expanding space-time which is isotropic and homogenous on very large-scales. But it is only a coincidence that in such conditions the geodesic of a test particle is described by the FLRW metric whose space and time components are independent so that the metric easily allows for an expansion factor while preserving the spatial geometry.

One has to remember that the FLRW metric is only relevant to test particles at very large scales where the cosmological principle is relevant. On smaller scales the equivalence principle of the special relativity requires the metric of freely-falling observers to follow a Minkowski metric which is void of any expansion parameter. Therefore it is utterly misleading to ask whether the earth is receding from the moon or if we will become taller as the universe expands as such assumptions don’t apply to our coordinate system. Thus asks Peter Coles mockingly, “what about a black hole? Do you think there’s a Hubble flow inside one of those, struggling to blow it up!?”. In scales of the solar system, galaxies or clusters of galaxies the gravitational collapse has long surpassed the Hubble expansion, therefore such systems should be studied as bound systems when considering the effect of Hubble expansion.

One has to also note that the expansion of space is supposed to be due to a primordial push at the time of the big bang. As Peter Coles puts it, galaxies ‘started out’ receding from us. This is our best understanding of the early motion of the large-scale structures. The evidence for this comes from the inflation paradigm which sets the initial conditions of the universe. Furthermore, in the above discussion, the “spatial expansion” due to inflation ignores the “acceleration of spatial expansion” which is associated to the dark energy. It has been observed that the expansion is actually accelerating. This has been presently attributed to the force exerted by the empty space (vacuum) itself on the space-time. This could further complicate the arguments, but it’s only relevant in even much larger scales!

In the Dark

A blog about the Universe, and all that surrounds it

Is Space Expanding?

I think I’ve just got time for a quick post this lunchtime, so I’ll pick up on a topic that rose from a series of interchanges on Twitter this morning. As is the case with any interesting exchange of views, this conversation ended up quite some distance from its starting point, and I won’t have time to go all the way back to the beginning, but it was all to do with the “expansion of space“, a phrase one finds all over the place in books articles and web pages about cosmology at both popular and advanced levels.