What if…? ‘Dark Energy’ was actually… well… light?
“Don’t be daft!” I hear you say… bear with me a moment… but first:
What is Dark Energy?
Well… in 1998 we had a shock revelation, when measurements attempting to bring some precision to the value of the “Hubble Constant” led to an unexpected result. One of the original questions had been along the lines of “How much is the Universe’s expansion (legacy of the Big Bang) slowing down (due to gravity)? – is it enough to make it collapse again in a Big Crunch, or will it continue to expand as the gravity is weakened by distance?”
The answer came back: well… it seems to be accelerating… not slowing down at all!
No-one knew (or knows) how or why ; but we all agree that acceleration requires a transfer of energy.
The source of this energy is highly contentious, but since we didn’t know where it comes from, or what form it takes, it was nicknamed ‘Dark’ Energy (in much the same way that Africa was ‘The Dark Continent’; dark as in unknown; yet to be revealed).
We have plenty of evidence (I’m trying to avoid the word ‘know’ here…) that the Universe is expanding, and the bulk of this evidence comes in the form of ‘Red Shift’ of the light (or any electromagnetic radiation) coming from extremely distant objects. The further away they are, the stronger the shift.
I’m not disputing this; it goes along with Einstein’s assertion that c (the speed of light in a vacuum; ~3 x 108 m/s) is the ‘speed-limit of the Universe’ – which has been tested repeatedly over the last century, and still stands un-contradicted.
If a light-wave travelling through a vacuum does indeed have a fixed speed, then because of the Doppler Effect, you would expect light from an object moving rapidly away from us to be ‘red-shifted’ whilst light from an object moving rapidly towards us would be ‘blue-shifted’. And, in fact this is what we see, suggesting that Einstein was right about this.
It’s generally a fairly subtle effect unless the object emitting the light is extremely distant, but because the whole Universe is expanding in between things, the further part two objects are; the more quickly they are moving away from each-other. Another place it is readily observed is in the spin of a spiral galaxy seen edge-on: one side (spinning towards us) is blue-shifted, whilst the other side (spinning away from us) is red-shifted.
What is the Doppler effect?
Whilst it applies equally to light, it’s perhaps easier to understand with sound waves:
Sound-waves have a fixed speed in air (roughly 330m/s assuming constant/standard conditions, etc.) with the result that if the source of a sound is moving towards you the sound can’t go any faster, so it still travels at its fixed speed (the speed the source is travelling at does not get added on to the normal speed of sound). Instead the sound changes not its speed, but its pitch (or frequency); the sound you hear will be higher-pitched as that object approaches you and lower pitched as it moves away.
So if you stand at the kerb as a moped approaches and passes you, you’ll hear the classic “neeee-ooooow”, as it switches from high to low pitch.
The frequency of the vibrations of the engine don’t actually change (assuming the throttle is kept steady), but as it approaches, the engine is slightly closer to you when the next peak leaves it, meaning that the peaks are slightly closer together than they would have been from a static engine. Closer peaks with the same wave speed gives higher frequency and shorter wave-length. Similarly, if it is moving away from you, the wave is stretched out as the engine emitting the pulses is slightly further away when each pulse is emitted, so the peaks are further apart and the pitch is lower.
With light it is not pitch which changes, but colour. The reduced frequency of light from an object moving away from us results in a reddening effect, or ‘Red Shift’, while objects moving towards us will show a ‘Blue Shift’.
But what has this got to do with Dark Energy?
Well, ordinarily we might assume, on timescales of less than a Million years, that a photon travelling (by definition!) at the speed of light, interiorly ‘experiences’ total time dilation, and so remains unchanged during its journey, having the same energy at the end as at the start.
Only when the photon is absorbed by something does it stop and release/deliver its energy.
But Einstein also said that the energy of a photon was proportional to its frequency (E = hf where h is Planck’s constant and f is the frequency).
That means that if a light wave/photon is red-shifted, i.e. falls in frequency, then it also loses some of its energy. The longer it travels through space, the more red-shifted it becomes and the more energy it loses.
Light has a Half-Life!
When we are able to measure the extent to which the light from an object has been red-shifted, Hubble’s Law allows us to calculate its distance from us, in light-years. But light-years are also a measure of time, so the same formula can also be rearranged and used to work out how long it takes for a photon’s frequency (and therefore its energy) to be halved.
That means we can work out a half-life for the energy of a photon (which applies as long as it is travelling through ‘empty space’). This comes out at approximately 7 Billion years (it depends on what value you use for the Hubble constant, which is highly contentious).
Given that the origin of the Universe (or at least the Big Bang) is taken to have been almost 14 Billion years ago, the frequency of the Cosmic Microwave Background is about ¼ what it was when those photons were originally emitted. Likewise their energy has dropped to ¼ of what it was.
But... Where does the Energy go?
The first law of thermodynamics tells us that energy cannot be created or destroyed, it can only change form: so where does the energy lost by the photons/light waves go?
And what is the mechanism by which it is transferred?
To the best of my knowledge Physics doesn’t have answers to either of these seemingly simple questions.
Now, if it were just a ‘few’ photons we were talking about, perhaps we could overlook this. However, most of Space is ‘empty’. This means that any photon successfully leaving its star of origin is highly unlikely to ‘hit’ anything. Ever.
Given just how empty space is, probably less than 1% of the light emitted by all the stars in all the galaxies in all the Universe actually get to ‘deliver’ their energy payload.
The other 99% endure and just continue to drift through space, taking their energy with them, locked inside a frozen time-capsule forever.
Except they don’t.
True, vast quantities of energy must be tied up in these stray photons, and it takes a LONG time, but they do relinquish their energy even if they never ‘hit’ anything. Over a timescale of around a Trillion years a half-life of 7 Billion years will reduce the energy of even the most energetic photons to close to zero.
The usual explanation is that they suffer cosmological red-shift ‘due to’ the expansion of space itself while they are travelling through it.
And as I have pointed out, as a consequence of this they fall in frequency and lose energy.
But how does this work? What is the mechanism by which energy is transferred from the photons? And what is the cause of the expansion in the first place?
Dark Energy...
But we have no idea what that is.
So on the one hand we have an unknown but vast source of energy, causing acceleration of the Universe’s expansion; and on the other hand we have an unknown mechanism by which energy is drained from a vast number of ‘lost’ photons.
Wouldn’t it be simpler to imagine that these are the same thing, but we’ve put the cart before the horse?
What if photons travelling through Space somehow interact with it (very weakly), but in the process transfer energy to it, causing the expansion which is reflected in the increasing wavelength.
The photons don’t suffer the cosmological red-shift; it is their transfer of energy and their increasing wavelength which causes the expansion in the first place.
And as the stars churn out ever more photons and the Universe expands and becomes even emptier, more and more of the photons become ‘lost’, transferring their energy to the expansion of space in ever-increasing numbers: so, naturally the expansion is accelerating…
So… Am I mad?
Let me know what you think!
