I've always figured there must be a simpler explanation for the observed lack of red shifting that has lead astronomers to hypothesize Dark Energy. Or at least there must be a simpler explanation of Dark Energy than an ever-increasing amount of energy that pushes the universe apart.
However, now that we observed the GW170817 event in two independent ways, does this confirm that Dark Energy exists, or is there way too much uncertainty to use it to make inferences with?

Although light is often said to be weightless this is simply not true because photons have momentum and therefore bend spacetime. I had hypothesized that at a quantum level the photons could interact with their own gravitational bow wake in such a way as to stretch the waveform out, possibly while destructively interfering with the waves in front and behind it, thus making it appear as if the energy had been stretched out over time.
However, now that we have observed a gamma-ray burst from both the light and the gravitational waves emitted, does this disprove the idea that gravity could be the cause of the redshift?

An alternative explanation to both Dark Energy and my hypothesis would be that dark matter or something else in the galactic medium, just has a red shifting effect on light waves.
But if both the gravity wave and lightwave diminished in intensity exactly as expected, and that nothing affected the light that didn't affect the wave; wouldn't that then prove that they were in fact just as far away as we expected them to be? I doubt that we have enough information to make such expectations, but if so I might have to start accepting Dark Energy.

EDIT: Originally, I had said extra redshifting instead of lack of, but I've since realized that if universal expansion appears to be accelerating, then more distant objects have slightly less red shifting then what would be expected of them.

  • $\begingroup$ "Dark energy" is required for the cosmological model by Alexander Friedman to fit observations. If you choose to believe that this model is correct, then you must admit that there exists mysterious "dark energy" that no one has ever seen, whose origin is unknown, and that is not needed for or justified by anything else. $\endgroup$
    – safesphere
    Oct 17, 2019 at 19:56

2 Answers 2


Your question is a mishmash of unrelated and incorrect ideas. Here're some of them:

  • The observed "redshifting" has been around since Hubble first measured it in the early 20th century. That was before the discovery of dark energy.
  • The current simplest explanation of dark energy is the cosmological constant, which is a constant, i.e. it is not an "ever increasing energy source".
  • Light is massless. The source you quote says as much. First five words: "The short answer is "no" ..."
  • The idea that something else is redshifting light is called tired light, which has become increasingly fringe because of a variety of observations.
  • Do you even know why we think dark energy exists? It's not just because of the redshift, or Hubble would've discovered dark energy. See Wiki.
  • $\begingroup$ Sorry, I edited the question to say extra red shift. I am not saying that I doubt there is no expansion of the universe period, just that there does not seem to be much evidence to suggest that it is accelerating. I'll admit that on the second point of the Wiki you gave me talks about the shape of the universe, whose relationship to dark energy I am not familiar with. On the topic of the waveforms and mass density (the third point of the Wiki) all of these calculations are based off of a universe whose growth has been accelerating, meaning they are invalid if it has not. $\endgroup$
    – Caston
    Sep 20, 2019 at 3:18
  • $\begingroup$ Granted I did not know about the 2011 study mentioned on the wiki, and if the study or the stuff done with the Large scale structure theory was redone I guess it is not likely that the 70% mass energy gap would disappear. $\endgroup$
    – Caston
    Sep 20, 2019 at 3:26
  • $\begingroup$ Dark energy breaks the law of conservation of energy, supposedly it's fine because entropy doesn't decrease, (that is supposedly it cannot increase entropy). Finally, photons have momentum (E^2 = (mc^2)^2 + (pc)^2 => E = pc) which means they bend spacetime. In fact theoretically light alone could create a black hole (called a kugelblitz). I deleted the link though because it was confusing. $\endgroup$
    – Caston
    Sep 20, 2019 at 3:54
  • $\begingroup$ @Caston I think the point here is that there's a lot of material you need to study before you can make sensible statements about this. For example you write that in the third point of Wiki all calculations are based off a universe whose growth has been accelerating. That's incorrect. See "...the voids were used as standard rulers to estimate distances to galaxies as far as 2,000 Mpc (redshift 0.6), allowing for accurate estimate of the speeds of galaxies from their redshift and distance". Do you know what a standard ruler is? If you don't, you can't begin to understand the measurement. $\endgroup$
    – Allure
    Sep 20, 2019 at 5:10
  • $\begingroup$ As for dark energy and conservation of energy, see: physics.stackexchange.com/questions/33404/… Why does the metric not being time-invariant imply energy isn't conserved? What is the first law of thermodynamics and how is it different from the law of conservation of energy? Again there are a lot of things one needs to know before one can make sensible statements about this. $\endgroup$
    – Allure
    Sep 20, 2019 at 5:14

Please note that a photon mass is not necessarily the most economical hypothesis to work with. First you would have to establish that the effect you want to predict is different for different massless fields. If photons turn out to require the same effective "cosmological mass" than gravitons, you would have to reassess if you can think of your expected effect as some residual mass, or actually as a geometric correction that affects all null geodesics


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.