Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: benm on 14/05/2019 16:00:48
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Izak has a great question:
Could dark matter simply be light photons either moving very slow or stationary? This would account for the gravity needed to keep things together and causing the expansion. If gravity can cause light to bend, surely over the billions of years light travels it can be slowed down.
Can anybody help illuminate things?
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I'm currently a Naked Scientists intern, and this happens to be right up my alley (my day job is dark matter detection). Here are my thoughts - feel free to chip in yours!
Some people DO think massive photons could account for dark matter - these are sometimes called "hidden sector photons" or "dark photons".
These would be new particles, similar to regular photons, but with some mass. There are a handful of experiments looking for these things - check out "DM Radio" for more: https://irwinlab.sites.stanford.edu/dm-radio-searching-axion-and-hidden-photon-dark-matter
It is important to note that these particles (if they exist) are not regular photons which have slowed down due to gravity.
Since photons have zero mass, they don't "feel" gravity the same way, for example, a tennis ball being thrown across a room does. That is to say, they don't feel gravity as a force which pulls them around.
Photons see the actual curvature of spacetime due to the mass of other things, and travel around on this curved space time - which is why light bends around stuff sometimes.
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Izak has a great question:
Could dark matter simply be light photons either moving very slow or stationary? This would account for the gravity needed to keep things together and causing the expansion. If gravity can cause light to bend, surely over the billions of years light travels it can be slowed down.
Can anybody help illuminate things?
As already mentioned, light does not respond to gravity in the same way as "massive" objects. If you throw a tennis ball upwards, it will slow as it climbs away from the ground, it is is giving up kinetic energy in exchange for a change in gravitational potential. So if you were tossing the ball to someone higher than you, he would measure the ball as moving slower when it gets to him than you initially threw it.
With light it is different. The light will give up energy climbing against gravity, but it will not show this as a slowing down. Instead, it exhibits it as a shift in frequency. Thus if you shine a light upwards to someone higher than you, then they will measure the light as still traveling at c when it reaches them, but they will measure it as having a lower frequency than what you measured it as having when it left you. It will have been "red-shifted" (A general term meaning a shift to a lower frequency, as light at the blue end of the visible spectrum has a higher frequency than the red end). Such a shift caused by climbing away from gravity is known as "Gravitational red shift". (light falling in towards a gravity source would be blue shifted.)
So there is no way to "slow down" normal photons so that they would orbit a galaxy the way that dark matter would. Light can have its path changed by gravity, but never its locally measured speed. There is only one place where gravity is strong enough to hold light in an orbit, and that is the photon sphere of a black hole, which is only 1.5 times the radius of the event horizon.
You also brought up "causing the expansion". The extra gravity holding galaxies together and the driving force behind the expansion are separate things.( To be clear, there doesn't have to be a driving force just for the universe to be expanding, the driving force is needed to explain why that expansion is accelerating over time.) While the term presently used for that driving force is "dark energy", and thus contains the word "dark" just like "dark matter", this doesn't mean that they are in any way related to each other. It's just a label.
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I actually though about it some time ago :)
Wondering if 'virtual photons' could be it.
But I don't think so
Dark matter and dark energy seem to exist.
Maybe it's a 'background'?
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If it is Einstein didn't make a mistake. Then you have to move it to the idea of 'constants', 'things' that just are.
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Naked Scientists intern...my day job is dark matter detection
Excellent job on the "Dark Matter" themed podcast this week!
I especially enjoyed the interviews with the two researchers; if I heard correctly, one was looking for very heavy particles (WIMPS), while the other was looking for very light particles (Axions).