What will eventually happen to all EM waves / photons?

  • #1

Summary:

What will happen to EM waves / photons when the rate of expansion of the Universe overtakes light speed?

Main Question or Discussion Point

I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me.

Going by the Lambda-CDM model, the expansion of the Universe will eventually overtake light speed, and all matter will disintegrate including sub-atomic particles. This part and it's subsequent conclusions are very unclear to me and I'm laying down my doubts in the following series of questions-

1) What will happen specifically to each of the 12 fundamental particles?

2) Just talking about photons, as the Universe is predicted to become a sea of isolated photons upon reaching maximum entropy, how will they be affected?
(I think we are somewhat better equipped to answer this one taking into account the effects we can already observe, i.e. redshifting.)

3) Supposing that an observer stands at the required distance from the center of the universe, will the first light of the Universe (idk, maybe the radiation from the Big-Bang) ever be able to make it to the observer or will it eventually get overtaken by expanding space? If it does, how far would that distance be?
(Asking this one because we roughly already know both the age of the universe and the time it will take for the rate of expansion to exceed light speed {the rate of acceleration of expansion, basically}.)

4) What will happen to exotic/strange matter? (Like the stuff that generated from strangelets near black-holes.)

And a related bonus question-

5) Should dark-energy, the causative agent of expansion, be considered as a 5th fundamental force of nature, since its effects are similarly independent of the other 4, and are already observable as well? (Maybe less than 4 but I don't wanna go into unification here.)

I understand that most of my queries fall into the realm of speculation and are more hypothetical or maybe even fundamental in nature, but I'd still like to know how much we can answer with our current knowledge.

Kindly explain your thoughts to me in similar layman-terminology about all this.

Thank you for your time.
 

Answers and Replies

  • #2
Bandersnatch
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Hi, @Yashraj Singh and welcome to PF.

It will be difficult to give you satisfactory answers, since your post indicates a number of major misunderstandings of the LCDM model. Those misunderstandings make the questions largely moot.
E.g.
- there is no centre of expansion in LCDM. The universe is modelled as homogeneous and isotropic, similar to the surface of a sphere or an infinite plane (but in three dimensions).
- it doesn't make sense to talk about the rate of expansion exceeding the speed of light, because there's no single speed to expansion. The rate is not a speed, so one can't compare it to a speed - the units won't match. Just by looking at Hubble's law one can see that any two sufficiently distant points are receding from each other with speeds arbitrarily higher than the speed of light.
- dark energy is not a 'causative agent' of expansion. It's causative of its acceleration, but expansion would happen without it (i.e. models with no dark energy are also expanding).
- the disintegration of matter that you mention is not a prediction of the LCDM. In it, the lambda bit means that dark energy has the form of a cosmological constant. The big rip scenario that you allude to requires dark energy to grow in strength over time.

Under LCDM expansion accelerates, but bound systems remain bound. This leads to heat death, not big rip.
Electromagnetic waves eventually become stretched to infinity (functionally disappearing), but fundamental particles do not disintegrate.
 
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  • #3
Hi, @Yashraj Singh and welcome to PF.

It will be difficult to give you satisfactory answers, since your post indicates a number of major misunderstandings of the LCDM model. Those misunderstandings make the questions largely moot.
E.g.
- there is no centre of expansion in LCDM. The universe is modelled as homogeneous and isotropic, similar to the surface of a sphere or an infinite plane (but in three dimensions).
- it doesn't make sense to talk about the rate of expansion exceeding the speed of light, because there's no single speed to expansion. The rate is not a speed, so one can't compare it to a speed - the units won't match. Just by looking at Hubble's law one can see that any two sufficiently distant points are receding from each other with speeds arbitrarily higher than the speed of light.
- dark energy is not a 'causative agent' of expansion. It's causative of its acceleration, but expansion would happen without it (i.e. models with no dark energy are also expanding).
- the disintegration of matter that you mention is not a prediction of the LCDM. In it, the lambda bit means that dark energy has the form of a cosmological constant. The big rip scenario that you allude to requires dark energy to grow in strength over time.

Under LCDM expansion accelerates, but bound systems remain bound. This leads to heat death, not big rip.
Electromagnetic waves eventually become stretched to infinity (functionally disappearing), but fundamental particles do not disintegrate.
Thank you for clearing that up.
Which scenario, as of now, do you think is more likely to happen and why?
And in the Big-Rip scenario, what does 'functionally disappearing' mean? And what does it leave the Universe as? A homogeneous sea of isolated fundamental particles only?
 
  • #4
PeterDonis
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Going by the Lambda-CDM model, the expansion of the Universe will eventually overtake light speed
This makes no sense. The expansion of the universe does not have a unique speed; it's described by a fractional rate of expansion, i.e., the universe expands by a certain fraction per unit of time (currently the fraction is about ##2.3 \times 10^{-18}## per second).

all matter will disintegrate including sub-atomic particles
This will never happen in the standard Lambda-CBM model. I think you may be confusing that model with the "Big Rip" model.
 
  • #5
This will never happen in the standard Lambda-CBM model. I think you may be confusing that model with the "
Yes, thanks to @Bandersnatch I know that now. I thought that Big-Rip was just one among many scenarios within the ΛCDM, not a whole different model.
Basically, I want to know what we can predict of the end result in the Big-Rip model, in terms of it's effects on fundamental particles and especially photons / EM waves, and the final composition of such a Universe.

And which model/scenario, according to you, is most likely to occur based on current knowledge, and why?

The expansion of the universe does not have a unique speed; it's described by a fractional rate of expansion, i.e., the universe expands by a certain fraction per unit of time (currently the fraction is about 2.3×10−182.3×10−182.3 \times 10^{-18} per second).
Also, this is a bit hard to understand for me. @Bandersnatch mentioned the likeness of expansion to that of the surface of a balloon, only in 3D. Can you please explain this 'fraction', and more specifically, it's use in the Big-Rip model?
 
  • #6
PeroK
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Yes, thanks to @Bandersnatch I know that now. I thought that Big-Rip was just one among many scenarios within the ΛCDM, not a whole different model.
Basically, I want to know what we can predict of the end result in the Big-Rip model, in terms of it's effects on fundamental particles and especially photons / EM waves, and the final composition of such a Universe.

And which model/scenario, according to you, is most likely to occur based on current knowledge, and why?


Also, this is a bit hard to understand for me. @Bandersnatch mentioned the likeness of expansion to that of the surface of a balloon, only in 3D. Can you please explain this 'fraction', and more specifically, it's use in the Big-Rip model?
You could try this insight. It may answer a lot of your questions:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
 
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  • #7
Bandersnatch
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Which scenario, as of now, do you think is more likely to happen and why?
And in the Big-Rip scenario, what does 'functionally disappearing' mean? And what does it leave the Universe as? A homogeneous sea of isolated fundamental particles only?
As was mentioned, in LCDM the end scenario is heat death. Whether it will happen depends on whether the LCDM model turns out to be correct.
John Baez wrote some musings about extrapolating it to extremely far future, that you may find right up your alley: http://math.ucr.edu/home/baez/end.html

By functionally disappearing I meant the wavelengths of any radiation become so redshifted that it ends up undetectable. You could make an argument that it's still there, but you'd need a telescope larger than the size of the observable universe to detect it.
It happens in any model that expands forever, not just big rip.

I thought that Big-Rip was just one among many scenarios within the ΛCDM, not a whole different model.
Think of big bang as a single theory that has some free parameters. Depending on what you set those parameters to be, the theory produces different models of behaviour of the universe. If you set matter density too high, it will recollapse. If you make matter density 'just right' or lower, it'll expand forever in a decelerating or coasting manner. If you add a dark energy component - it'll expand exponentially. If you make the dark energy grow over time, without bound - the expansion rate will grow to infinity.
You try to find out what the parameters are by observations, and currently the LCDM looks kinda like it fits those observations best. But there's still enough uncertainty to call it provisional. Also, some scenarios are pretty much ruled out (like recollapse).

Also, this is a bit hard to understand for me. @Bandersnatch mentioned the likeness of expansion to that of the surface of a balloon, only in 3D. Can you please explain this 'fraction', and more specifically, it's use in the Big-Rip model?
Fractional rate of expansion is the meaning of the Hubble constant. The value of approx 70 km/s/Mpc (notice that the units are actually 1/time) translates to: all distances grow by 100% every 14 billion years. Or by 1/140th of 1% per million years.
Now, the naming in 'Hubble constant' is a bit misleading, specifically the 'constant' part. It's only constant across space, and it can be constant in some models, but in general it does change with time.
(To be precise, one should use 'Hubble constant' to denote its value today, and 'Hubble parameter' for the time-variable version)

Specifically, in a universe without dark energy, it always goes down. This is in part due to the decelerating effects of matter, but mostly due to simple proportionality: if you imagine a galaxy with some recession velocity, and even if you assume there's no other matter in the universe that could decelerate this velocity, the rate of growth of the distance between you and it will be going down with time. I.e. let's say it's initially 10 billion light-years (Glyr) away, and with its recession velocity it takes 10 billion years (Gyr) to double this distance (not the actual numbers). After 10 Gyr it's going to end up 20 Glyr away with the same speed. So in another 10 Gyr it will cover another 10 Glyrs, which as a fraction of its total distance is no longer 100% growth, but 50% growth per the same amount of time. Which means the Hubble constant went down by half.
Matter content (and radiation too, but negligibly now) cause the initial speed to fall with time, so the fractional growth after each interval is even lower.

In a universe with cosmological constant (and to simplify, otherwise empty) the Hubble constant stays constant. I.e., using the scenario above, every 10 billion years every distance always doubles. Which means every recessional velocity doubles too (=exponentially accelerating universe).

LCDM starts off like the first scenario, because the effects of dark energy are initially completely swamped by the decelerating influence of matter. But eventually dark energy takes over, and the fractional growth asymptotically approaches a constant value in the far future.

In big rip, the fractional growth itself grows, with the size doubling time getting ever shorter. This can be interpreted as an ever stronger tidal force pulling everything apart. So any system that is initially bound by some attractive force, be it gravitational, electromagnetic or nuclear, eventually loses cohesion.

I don't know enough about how elementary particles behave under such conditions to go any further. I imagine it all eventually decays to radiation, and that radiation is then stretched to non-detectability.


Lastly, here's some more reading to add to @PeroK 's recommendation:
(these two cover a number of essential basics in layman-friendly terms)
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdfhttps://www.physicsforums.com/insights/balloon-analogy-good-bad-ugly/(this one is a more involved tutorial on a level similar to PeroK's link)
http://www.astro.ucla.edu/~wright/cosmolog.htm
 
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  • #9
Thank you again, @Bandersnatch.
Now that I think about it, my original confusion between the LCDM's heat-death outcome and the Big-Rip's outcome wasn't unfounded - it's pretty common actually among various circles as I have observed, given the similarities in the end results of both outcomes and their popular interpretations available.

So as I understand it now - basically, both the heat-death and the Big-Rip are driven towards reaching a maximum entropy state, and in both, the Universe ends up ultimately as a sea of isolated photons and elementary particles resulting from decay of matter, with black-holes being the last penultimate structures. The difference between the two being that in the Big-Rip model, dark energy continues to increase and with it, the rate of expansion, and that all the remaining radiation common to both models is infinitely stretched in the Big-Rip. In addition, the hypothetical timescale of the Big-Rip is exponentially less than that of the heat-death by vast orders of magnitude.
I know this is a very crude way to put it, but please tell me if this interpretation is somewhat close to correct or if it misses the mark entirely.

Also, I haven't been able to find any resources broaching the subject of the effects of expansion on strange/exotic/virtual matter, even hypothetically, in the current Universe, and in the case of a runaway expansion (Big-Rip or heat-death) in which ordinary matter decays.
Any information regarding this topic will be much appreciated.
 
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  • #11
PeterDonis
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both the heat-death and the Big-Rip are driven towards reaching a maximum entropy state
No. In a universe that expands without bound there is no maximum entropy state; entropy can continue to increase without bound.
 
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