I Dark energy questioning Relativity?

Summary
Dark energy seems to have been invented because observations don't fit in with General Relativity. So why is the theory itself never questioned?
Forgive my naive understanding of these topics. I have a layman's interest in science and follow all the popular science I can, but I'm certainly not a physicist and even my degree in mathematics is now all but forgotten in the past.

But I have two questions (the other I'll post in another thread) about astrophysics as presented in the popular press that I can't get out of my head. I wondered if you folks could save me a lot of time and probably futile research, as these topics are most likely way beyond my current understanding.

All the reasons for the (proposed) existence of Dark Matter (and Dark Energy) seem to be based on gravitational effects. Galactic clustering. Galactic rotation. The CMB. Large-scale clumping. These observations don't fit with our current understanding of gravitation (General Relativity) so Dark Matter is invented, as this seems to explain things.

Is that a fair summary?

In that case, what about the famous explanation of the Scientific Method by Richard Feynman: "If it disagrees with experiment, [the theory is] wrong. In that simple statement is the key to science."

I buy into the whole 'General Relativity is great, it predicts everything and is the best model we have' etc. shtick, but Newton was completely valid up until someone noticed something funny about the orbit of Mercury and then GPS and then galactic lensing and then black holes crashing into each other and so on.

Something like Dark Matter seems invented despite complete lack of evidence for it and I'm kind-of fine with that. But why in light of the above is the possibility that General Relativity may simply not be correct/sufficient in some cases never discussed at all?

Thanks,

P
 

Nugatory

Mentor
12,261
4,710
But why in light of the above is the possibility that General Relativity may simply not be correct/sufficient in some cases never discussed at all?
There's an analogy with the observed orbit of Uranus. By the early 19th century Newtonian gravity had proven exquisitely successful at calculating everything from the orbits of the heavenly bodies to the trajectories of dropped or thrown objects. However, it failed to properly describe the orbit of Uranus. Thus, there were two possibilities:
A) Newtonian gravity is not correct/sufficient. We should put our intellectual energy into discovering a new theory that agrees with Newtonian gravity everywhere that Newtonian gravity works but incorporates some as-yet-undiscovered new physics that is unnoticeable near the sun but at greater distances has exactly the right effect on the orbit of Uranus.
B) There's nothing wrong with Newtonian gravity. We've just left something out of our calculations, and we should put our intellectual energy into finding that something.

Phrased that way, it's clear that #A was a wild longshot, not worth much discussion unless and until #B could be excluded. So astronomers hypothesized that there was some undiscovered mass out here, calculated where it would have to be to affect the orbit of Uranus, and then in 1846 pointed their telescopes at that spot.... And there was the planet Neptune!
(This is, of course, the same thought process you go through when you pick up a box and find it harder to lift than you expected. Your first thought is not that Newtonian gravity is wrong about the earth's gravitational pull on the box, it is that there's something heavy that you don't know about in the box)

With regard to dark matter and dark energy (which, despite sharing the word "dark" are completely different things) right now we're at the "finding that something" stage. We can assume that general relativity is wrong, or we can assume that we haven't observed everything that's out there to observe; and the latter looks like a much better bet.
 

Orodruin

Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
15,844
5,842
On the other hand ...

**Edits in bold**
There's an analogy with the observed orbit of Uranus. By the early 20th century Newtonian gravity had proven exquisitely successful at calculating everything from the orbits of the heavenly bodies to the trajectories of dropped or thrown objects. However, it failed to properly describe the orbit of Mercury. Thus, there were two possibilities:
A) Newtonian gravity is not correct/sufficient. We should put our intellectual energy into discovering a new theory that agrees with Newtonian gravity everywhere that Newtonian gravity works but incorporates some as-yet-undiscovered new physics that is unnoticeable near the sun but at greater distances has exactly the right effect on the orbit of Mercury.
B) There's nothing wrong with Newtonian gravity. We've just left something out of our calculations, and we should put our intellectual energy into finding that something
The correct approach was A and led to general relativity.
 
28,141
4,531
Summary: Dark energy seems to have been invented because observations don't fit in with General Relativity. So why is the theory itself never questioned?

Newton was completely valid up until someone noticed something funny about the orbit of Mercury
Actually, the situation with Newtonian gravity is more similar to GR than you may realize. As the precision of measurements increased there were discrepancies noticed in the orbits of Jupiter and Saturn. Solving Newton’s equations for the known sources resulted in a small failure. So they had two options: modify the equations or modify the the sources.

The equations seemed to work well elsewhere so they tried predicting the sources that would explain the observed discrepancy. This led to the discovery of Neptune and Uranus and eventually Pluto and other objects.

A similar scenario happened with the discovery of the neutrino. A discrepancy in the conservation of momentum and energy was noticed in nuclear reactions. They could abandon the laws of conservation of momentum and energy, or look for unknown sources. The laws of conservation of energy worked well in other scenarios so they decided to look for unknown sources. Looking for unknown sources they found the neutrino.

If the laws of physics that are known to work well in many scenarios have a small discrepancy, the first thing to do is to look for additional sources. This is what the search for dark matter is.

Of course, as you mentioned, sometimes we need to change the theory, like in the case of Mercury. Unfortunately, it is not possible to know in advance whether a given discrepancy is a new-source discrepancy or a new-equation theory.

Certainly, when a new theory is developed we apply those equations to the discrepancy using the known sources. But this presupposes the existence of a new theory. GR was not developed to explain Mercury’s orbit, it arose due to other considerations. Currently we don’t have a viable alternative.
 

Nugatory

Mentor
12,261
4,710
On the other hand ...
I was considering writing a footnote about Mercury's orbit for exactly that reason - it is both the same and different, in interesting ways.

The big difference is that the incompatibility between special relativity and Newtonian gravity was apparent and troublesome before the discovery of GR - we had two theories and they could not both be correct. Thus, there was reason to pursue the #A line of thought even in the absence of any anomalous observational data.

As the history played out, the anomalies in Mercury's orbit did not motivate general relativity (and it's not at all clear that one could arrive at GR by studying them). At the time there was no particular reason to think that the Mercury problem was related to the theoretical problem of reconciling special relativity and gravity. Instead, the flow went the other way: GR was discovered for other reasons and only then could we say "Wow - so that's what's going on with Mercury".

(Another footnote on the relationship between observation and new theory might be Fizeau's 1850-vintage measurements of the speed of light in a medium. These went unexplained until Einstein discovered special relativity a half-century later).
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,094
5,385
Summary: Dark energy seems to have been invented because observations don't fit in with General Relativity. So why is the theory itself never questioned?
It's a pity that the first responders here completely swallowed your premise, which is simply not true. Alternatives to GR are developed and considered all the time. There is so much work there needs to be review articles summarizing this work, e.g. Annu. Rev. Nucl. Part. Sci. 66:95-122 (2016) which references one-hundred-and-fifty-four papers.

Before we can discuss why something is true, we need to know if it is true.

It's not.

Something like Dark Matter seems invented despite complete lack of evidence for it
Untrue.

But why in light of the above is the possibility that General Relativity may simply not be correct/sufficient in some cases never discussed at all?
Untrue.
 

Ibix

Science Advisor
Insights Author
5,196
3,507
There's also the Pioneer Anomaly, where some of our deep space probes were drifting off course. That turned out to be an unanticipated thrust effect from the vehicle's reactor interacting with the antenna. So a third possibility is some known physics not properly accounted for. No idea what kind of thing that might be in this case, but I mention it for completeness.

It's also worth noting that there are alternative models of (non-quantum) relativistic gravity. It's just that they keep getting ruled out by experiment. I seem to recall LIGO dealt a solid blow to a number of theories that allowed gravitational waves that aren't pure transverse-tensor waves, since the detected signals weren't consistent with other types of wave.
 
Last edited:

PeroK

Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
9,667
3,575
Summary: Dark energy seems to have been invented because observations don't fit in with General Relativity.
Your conclusion is false in another sense. GR, in its most general form, works with whatever energy sources the universe has. A simple application of the theory assumes energy from matter and radiation and assumes a zero energy of the vacuum.

But, one explanation for an accelerating expansion is that the vacuum has non-zero energy.

The theory of GR, per se, is unchanged. It's the addition of the vacuum as an energy source that leads to the accelerated expansion, by directly applying the equations of GR.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,094
5,385
On the other hand ...
Yes, but the first hypothesis for the orbit of Mercury discrepancy was a new inner planet, Vulcan. Not crazy, since gravitational effects from other planets were 13x larger than the discrepancy: a small body is about the right magnitude. However, nothing was ever found here: people looked from 1843 (first calculation) to at least 1908.
 
26,240
6,860
"If it disagrees with experiment, [the theory is] wrong. In that simple statement is the key to science."
"Experiment" is not the same as "observation". What Feynman meant by "experiment" here is something more like "observations and experimental data, after the observations and data have been thoroughly checked and we're sure they were all made correctly and that we've correctly accounted for all the observational and experimental errors, and the theoretical calculations have been thoroughly checked and we're sure there aren't any physical effects going on that we don't understand and we're sure that there aren't any other objects or sources out there that we haven't taken into account and we're sure that there aren't any other theoretical effects that we left out but should have been included, etc., etc., etc." If you read enough of Feynman, you will see him talking about all these pitfalls that come into play when you try to compare theory with experiment.
 
26,240
6,860
All the reasons for the (proposed) existence of Dark Matter (and Dark Energy) seem to be based on gravitational effects. Galactic clustering. Galactic rotation. The CMB. Large-scale clumping. These observations don't fit with our current understanding of gravitation (General Relativity)
No, that's not correct. All of those effects fit in fine with our current understanding of gravity (GR), with the appropriate sources (stress-energy) present, as @PeroK pointed out in post #9. The issue is that the sources we have to include in the model to match the data aren't all visible to us; we can't detect some of them, the ones we call "dark matter" and "dark energy", by any means other than their gravitational effects.

Many scientists are basically OK with that; they view part of what they are doing as just figuring out which sources need to be included in the model to match all the data. If some of those sources aren't detectable currently by other means, that's to be expected; there's no reason why our detection accuracy has to be the same in all regimes. They view including such sources in the model as much more likely to be correct than trying to change the underlying theory of gravity.

Other scientists are not OK with that; they feel that we should only include in our models sources that we can detect by other means. So if those sources can't explain the data, they want to change the underlying theory of gravity.

Both lines of research are being pursued, so, as @Vanadium 50 has pointed out, it's not correct to say alternatives to GR are not being investigated. We simply have not reached the point where either line of research has conclusively been either established or ruled out. This is a perfectly normal thing to have in a scientific field.
 

Want to reply to this thread?

"Dark energy questioning Relativity?" You must log in or register to reply here.

Related Threads for: Dark energy questioning Relativity?

Replies
5
Views
3K
  • Posted
Replies
11
Views
3K
  • Posted
Replies
6
Views
839
  • Posted
Replies
3
Views
5K
  • Posted
Replies
4
Views
305
Replies
1
Views
894
  • Posted
Replies
5
Views
2K
  • Posted
Replies
1
Views
474

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top