If the room expands evenly in all directions

Where is the universe expanding to?

According to the standard cosmological model and based on the spectral measurements of galaxies, space is expanding. “Where is the universe expanding to?” Asks our reader Bernhard K. via email - thereby expressing a widespread misunderstanding.

Albert Einstein's general theory of relativity is the theory for gravity, spacetime and the universe as a whole, which has been brilliantly confirmed by measurements to this day. It follows from it that space cannot be static on a large scale. It would be unstable, as Einstein recognized in 1917 (but initially refused to admit). So the space has to contract or expand. Measurements of the spectral lines of distant galaxies and galaxy clusters as well as further observation data show that the universe is expanding. Curiously, it has been doing this faster and faster for about six billion years.

Three misunderstandings

There are three common misconceptions:

  • The universe expands from a kind of center point that marks the Big Bang.
  • Since then, space has been expanding equally everywhere.
  • The universe expands at its “outer edge”.

All of these statements are false.

Four cosmological consequences

The following consequences of cosmology based on general relativity are correct:

  • The Big Bang was not an explosion in space and time, but an explosion in space and time. The spacetime of our universe only came into being with the Big Bang. So there is no center of the universe, rather the big bang took place everywhere.
  • Space only expands where the gravity of matter and energy is low. It therefore does not expand within galaxies or compact galaxy groups, but between the galaxy clusters and superclusters. The rate of expansion (misleadingly called Hubble constant) is also not constant in time, but changes in space and time. (This is why it is best to speak of local Hubble parameters and a global, time-dependent Hubble parameter as the average.)
  • There is no outer edge of the universe - regardless of whether it is finite or infinite. However, there are fundamental limits to observability for every place in the universe: the cosmic horizons. They are the result of the finite vacuum speed of light. So light needs time to cover distances. Therefore, every look out into the room is a look back in time. Because our universe has a finite age and is expanding, we can only look at part of the universe.

The concrete data: The age of the universe is around 13.8 billion years and its rate of expansion is currently around 70 kilometers per second and megaparsec (this is the Hubble “constant”). We can therefore look back around 13.8 billion years - the first light that can still be measured in the microwave range today was created 380,000 years after the Big Bang. We can not only peer out 13.8 billion light years into space, but further. Because it is not static, but has expanded and still does. How far we can see in principle depends on the expansion history of the universe. According to the latest data, our cosmic horizon is around 46 billion light years away.

  • The universe is expanding not only on the cosmic horizon (which is different for every observer anyway), but everywhere between the galaxy superclusters. The space does not expand “outwards” - or even within a space beyond the universe or into an additional “surrounding space”. There is no evidence for the existence of such an extra space, nor is it required in the cosmology of the theory of relativity. Rather, space expands “inwardly”. So it expands wherever gravity does not hold it together, and this expansion adds up over the distances (and at sufficiently large distances it even becomes effectively faster than light!). Gravitationally undisturbed stretches therefore increase: a route of one megaparsec length (that is 3.26 million light years) expands by around 70 kilometers every second.

Rüdiger Vaas is bdw editor for astronomy and physics. This text is based on the new edition of his book Hawking's New Universe (Kosmos-Verlag 2018).

If you also have a question or a suggested topic for our "Inquired" section, simply send us an email to :[email protected]

5th August 2018

© Wissenschaft.de - Rüdiger Vaas