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Q & A: Receding galaxies

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Most recent answer: 10/25/2009
Astronomers say that the universe is expanding and galaxies are receding from each other with an accelerating speed. What would happen if galaxies were to recede from each other with a speed greater than c? Or is it only the space between the galaxies that is growing with an accelerating pace? If so, this sudden extra space is due to dark matter as physicists say. Do we know if dark matter is able to expand with a speed greater than c since it does not seem to be bother with universal physical laws?
- Anonymous
First, let's get some of the observational facts down.  Then we can look at some of your questions.

1. During the 1920's and 30's, Edwin Hubble discovered that the Universe is expanding, with galaxies moving away from each other at a velocity given by an expression known as Hubble's Law: V = H*R. Here V represents the galaxy's recessional velocity, R is its distance away from Earth, and H is a constant of proportionality called Hubble's constant.

2. The astronomer Fritz Zwicky first proposed the idea of 'dark matter' in 1933 when he observed that the relative motion of galaxies within galactic clusters could not be explained solely by the presence of luminous matter, i.e. stars.  There had to be something else contributing to the gravitational forces.   Since that time more and more evidence has piled up in favor of this idea.  According to modern astrophysical observations the amount of the stuff exceeds that of luminous matter by a factor of ten!  There are various theories about the composition of dark matter, for example, a kind of elementary particle that interacts very weakly with ordinary matter.   Sensitive earth-based experiments are being carried out to try to detect the individual constituents of dark matter but up to now no 'smoking gun' has been found.

3. The idea of 'dark energy' is a relatively new idea although Einstein had a term in his famous general relativity equations called the 'cosmological constant' that could mimic the effect.   The recent observational evidence comes partly from looking at the excessive dimming of supernovae stars as a function of their distance, or red-shift, from the earth.  The effect  can be explained as an increase in the Hubble expansion rate as a function of the age of the universe.  This can, possibly, be explained by ascribing to space itself a property called 'dark energy'.   Itís pretty weird.  The idea is fairly new and many astronomical observations must be made in order to confirm it.

Check out the Wikipedia articles on Hubble, dark matter, and dark energy for more information.

Now the effect you are referring to in your question is probably related to the accelerated expansion rate due to the 'dark energy'.   'Dark matter', per se, doesn't violate any 'universal  laws', its gravitational interaction is well behaved.  The fact that we haven't seen any sign of it in our ground based particle detectors probably means that our detectors are not sensitive enough.  We're still improving them. Stay tuned.

As to the accelerated expansion rate due to 'dark energy' we will never see any far off galaxies going faster than the speed of light, c.   They just get dimmer and dimmer, and more and more red-shifted and then slowly fade out of sight, as T.S. Elliot said 'Not with a bang but a whimper'.  Pretty bleak, eh?


I beleive that, in an acelerating expansion, objects can ultimately actually depart across íhorizonsí, so that they not only become too dim to see but actually incapable of sending any signal that would ever reach us. Mike W.

(published on 10/22/2007)

Follow-Up #1: red shift analysis

If Galaxies are receding shouldnít the Doppler effect that is used for the velocity measurement be also supplemented by the shift of the spectral lines due to the inertial mass increase of electron in the Rydberg formula? Isnít the light that is observed emitted by the electronic transitions of excited atoms? If this is done, the universe is not flat.
- Jerry
Dallas,TX, US
Nice question. Simple answer: no.
The frequency of the atomic transitions in the rest frame of the atoms has to be the same as it would be for our atoms in our rest frame. Then the relativistic Doppler rule for transforming the frequencies to our  frame gives the standard answer, regardless of the detailed physics which went in to the original frequencies. Relativity is an amazingly powerful general tool!

There are a couple of adjustments needed in General Relativity. The Special Relativistic Doppler effect is only an approximation suitable for closer objects, and there can be gravitational red-shifts from more massive objects. These effects are of course routinely considered by the actual practitioners.

Mike W.

(published on 10/25/2009)

Follow-up on this answer.