Is light dark energy?

Question

I was wondering if it was possible for light itself to be dark energy, might sound silly but it's been breaking my brain for months and I can't find any reason why not. If light can interact with matter by around 1 billions of a pound times that by the size of the universe, could this be possible?

Answer 1

What makes dark energy special is the way it behaves as the universe expands.

Suppose you take regular matter like air, which at STP has a density of around 1 kg/m$^3$. If you expand the air to 2 m$^3$ then its density obviously halves. In general if you expand the air by a linear scale factor of $a$, i.e. a volume scale factor of $a^3$, then the density will be:

$$ \rho = \frac{\rho_0}{a^3} \tag{1} $$

This seems pretty obvious, but surprisingly other types of energy do not behave this way. Suppose we consider light, where by density we now mean the energy density i.e. the energy of the light per cubic metre. As the universe expands the energy density falls like matter, but in addition to this the light is red shifted by the expansion. This is known as the cosmological red shift. The red shift also decreases the energy of the light, and the end result is that the energy density of the light is given by:

$$ \rho = \frac{\rho_0}{a^4} \tag{2} $$

Note that we now have a factor of $a^4$ not $a^3$ in the denominator so as the universe expands the energy density of light falls faster that the energy density of matter.

Dark energy is different again. If it behaves like a cosmological constant then its energy density doesn't change at all with expansion:

$$ \rho = \rho_0 \tag{3} $$

This is highly unintuitive because it means that if we take some volume of space and let it expand, then the amount of dark energy inside that volume increases in proportion to the volume i.e. energy is appearing from nowhere. Yes, this does violate conservation of energy, but then conservation of energy doesn't apply to an expanding universe.

The key point about this behaviour that concerns us is that when the energy density is given by equation (3) it means the material behaves as if it had a negative pressure, and it's this negative pressure of the dark energy that causes the expansion of the universe to accelerate.

The point of all this is that light cannot be dark energy because its density is given by the wrong equation - equation (2) not equation (3). For the same reason matter cannot be dark energy either. Dark energy must be something that doesn't behave like matter or light. What that something is we simply don't know (yet).

Answer 2

No, light can't be dark energy. The contribution of light to the expansion of the universe has been taken into account in the calculations, and it's just too small. Dark energy is still needed to explain the accelerated expansion of the universe.

Answer 3

Leon mitchell's suspicion is correct. Space is simply light pressure, a spherical explosion of overlapping propagating electromagnetic waves. There is a direct correlation between the amount of EM that stellar objects have released, and the rate of inflation. During the dark period, inflation slowed, then, what do you know, as the EM output increased once again, inflation increased along with it. Anyone that simply shrugs off of light as being the cause of space or (A to B), is not grasping the fact that there is no other energy source that is as broad scoped and great enough to account for inflation aside from light(EM). When light is in wave form, it's footprint is much larger. The reason planets and stars end up with a spherical shape is because the light pressure that space is made of is nearly uniform and even from all directions, resulting in a balled up paper shape as space's pressure coaxes matter into clumps. We are held to the Earth because space(EM) is clumping us and the Earth together.

((("""Note that we now have a factor of a4 not a3 in the denominator so as the universe expands the energy density of light falls faster that the energy density of matter."""))) --- There are indeed regions of high density space and low density space. Next to a star the light pressure is great. In great voids, light pressure is less and thinner. Thanks to relativity, we would not notice minor differences in pressure densities, however, the ppm levels for nearly everything will be lower in the heart of voids, than within a light year of a star. Some stars are so massive that the outward light pressure is greater than gravity which prevents new material from being added to the massive star.

((("""Dark energy is different again. If it behaves like a cosmological constant then its energy density doesn't change at all with expansion:"""))) --- Space is not a constant. Liquid and gas can easily have pressure differences, even from simple compression waves. Take a blazar for instance, it is focusing an insane amount of EM in one direction for long periods of time. It is literally adding to space the most in that direction, yet, it does not effect physics or prevent matter or space from behaving like matter or space.

((("""This is highly unintuitive because it means that if we take some volume of space and let it expand, then the amount of dark energy inside that volume increases in proportion to the volume i.e. energy is appearing from nowhere. Yes, this does violate conservation of energy, but then conservation of energy doesn't apply to an expanding universe."""))) --- You are confusing dark energy(EM) as having more of itself simply because it is thinning out. Does water converted to gas thanks to pressure changes, contain more energy? No! Yet it has a larger footprint.

((("""The key point about this behaviour that concerns us is that when the energy density is given by equation (3) it means the material behaves as if it had a negative pressure, and it's this negative pressure of the dark energy that causes the expansion of the universe to accelerate."""))) --- So, when a region of higher density enters a region of low density, somehow, the low density behaves like a negative pressure,... interesting,... it is almost as if you are a proponent of light being space. You are scoring on the other team.

((("""The point of all this is that light cannot be dark energy because its density is given by the wrong equation - equation (2) not equation (3). For the same reason matter cannot be dark energy either. Dark energy must be something that doesn't behave like matter or light. What that something is we simply don't know (yet)."""))) --- I suspect you are unable to rationalize EM being space because EM is the only thing that propagates amongst itself as energy. EM is like water that is also the energy waves within itself. Light can behave like a particle or a wave, but water can't? Space is simply thinned out light. If you stick your hand out in any point of space, light is caught,.. hmm. Particles appear seemingly out of nowhere because two EM waves happened to collide causing a peak that appears as a particle. The Casimir effect happens because light pressure, however minute, pushes the plates together, and the broad sides of the plate catch the most light.

((("""No, light can't be dark energy. The contribution of light to the expansion of the universe has been taken into account in the calculations, and it's just too small. Dark energy is still needed to explain the accelerated expansion of the universe."""))) --- So how exactly has every little bit of EM output for the past 13+ billion years been accounted for in order to prove that space is not EM? After all, if space is an overlapping pool of EM waves, then, how are you able to detect the EM that is not pointing directly at you? 99.9999+++... of the EM is not heading in your direction. Light does not shine until it hits a detector.

Occam's razor,... the simplest explanation is normally the answer. Light is so present that it is easy to overlook. Please ask me anything.

!!!How about we continue this discussion because there is plenty for us to figure out. Let there be light on this subject. Our powers combined can result in an award winning paper and dark energy being solved.!!!