Dark energy/matter

  • #1
RK7
23
0
So as someone with no real knowledge of the topic, can anyone tell me what I'm missing?

Our observations don't fit our models - we see an accelerating universe, rotation curves which do not meet expectations etc
We decide there must be dark matter and dark energy to explain this and then try to find it.

Why don't we say that our theories of gravity are wrong? I sometimes see people quoting the accelerating universe and rotation curves as being evidence for dark energy and dark matter but surely they can't be evidence when their existence is inferred from the observations in the first place! It sounds circular to me. What am I missing?
 

Answers and Replies

  • #2
773
0
There are people who say our theories are wrong, and there are many people who have/are attempting to modify our current theories to fit the observations.

The reason the dark matter/dark energy model is widely accepted is twofold:
a) attempts to fix our current equations to suit observations have been mostly unsuccessful (if not completely unsuccessful)
b) our theories have already been extremely well verified, and everywhere our currents theories don't work, they would work if we accounted for dark matter/dark energy.

So in summary, we have extremely successful theories that fail in only a couple places, so the realization that only two changes need to be made to fit the highly successful theories to the data is more appealing that reformatting the entire theory.
 
  • #3
84
2
You are not missing anything.

When I read of 'dark matter' attracting observable matter on galactic distance scales and 'dark energy' expanding space on galaxy super cluster distance scales the EFE's (Einstein Field Equations) are firmly in my gun-sights.

Regrettably I don't have an alternative to propose but my 'gut' tells me something is rotten in our understanding.

I am also cautious because astrophysical observational data is still very very much front-line science (it is within living memory that we thought what was the whole universe we now know as our little galaxy)

I do like to recall the words of Sir Francis Bacon “If a man begin with certainties he will surely end with doubts but if he is content to begin with doubts he may end with certainties”

Keep thinking – I will

Regards

Sam
 
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  • #4
Chronos
Science Advisor
Gold Member
11,408
741
The case for dark matter is fairly compelling at this point. There are numerous observations of gravitationally bound systems that defy Newtonian gravity without dark matter, but, fall neatly into place with dark matter. There are gravitational lensing observations that insist on a huge amount of unaccounted for matter that are also simply resolved by dark matter. There is the CMB which requires an enormous amount of gravitating matter which cannot be accounted for without dark matter. And there are primordial elemental abundances that would be way out of whack without large amounts of nonbaryonic [dark] matter. It is true we have yet to pin down the elusive particle that comprise dark matter, but, is such an undeniably simple and effective solution to so many otherwise inexplicable problems in cosmology it has obvious appeal to the great majority of cosmologists. The case for dark energy is not so deeply rooted, but, it is still in its infancy having only been around for a dozen or so years. It too offers a simple solution to a number of problems. Bear in mind neutrinos were in the same boat for the better part of a century. There was great confindence they existed because they neatly solved some obvious problems in particle physics, but, they eluded actual detection for about 70 years.
 
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  • #5
319
0
Why don't we say that our theories of gravity are wrong? I sometimes see people quoting the accelerating universe and rotation curves as being evidence for dark energy and dark matter but surely they can't be evidence when their existence is inferred from the observations in the first place! It sounds circular to me. What am I missing?
The main reason we don't say the theories are wrong is because our observations do not presently rule out the fundamental physics we believe in. In the case of dark matter, there is not yet some observation which implies that we need to revisit our basic physics -- there is only an observation that there is some matter that we are not familiar with, but possibly may not require any new interactions to understand. In the case of dark energy, our observations generally seem to be consistent with general relativity, just with a particular value of a parameter in the field equations. So you are quite right in implying that there's no reason to be particularly confident that this method of inquiry will pan out, but surely it is worthwhile to engage in.
 
  • #6
1
0
For years, we seemed to have a good idea of how things worked on a large scale, and we seemed to have a good idea of how things worked on a small scale. Fitting the former with the latter in a unified theory seemed impossible, but it was okay because we didn't have to have a unified theory to explain how things worked.

Until improved methods of observation made our current understanding of how galaxies worked questionable. It seemed there wasn't enough mass in galaxies to account for their ability to stay together and not fly apart. So came theories of dark matter and dark energy. And everything would be okay except the new theories account for a majority of the universe!

When ideas about mass and gravitational attraction seemed solid, everything in the observable universe (for the most part) was accounted for and all was good. But when the bubble burst what did physicists do? They go and invent an answer, but at least things work now, right?

Why would not it be more feasible to look at the fundamentals of physics instead of invent an idea that in large part cannot be observed or tested in a lab? If the problem involved mass and gravitional attraction and how galaxies worked, let's look at the fundamentals which were laid down hundreds of years ago, people. Could it be that we need an update on what we assume to be the fundamentals of physics?

Obviously, our fundamental ideas about mass and gravity are lacking. I'd be willing to bet a unified theory will leave dark matter and dark energy at the wayside.
 
  • #7
773
0
I'd be willing to bet a unified theory will leave dark matter and dark energy at the wayside.
I agree with the idea, but I think its a bit trickier. My guess (and its just that- a guess) is that dark matter will be 'solved' by a modification of our galactic models or a small change in subatomic masses (like the neutrino having a small mass). Dark Energy on the other hand isn't so simple, my guess would be dark energy will be 'solved' when we figure out a good, testable reason why the cosmological constant is what it is.
 
  • #8
Drakkith
Staff Emeritus
Science Advisor
20,991
4,810
When ideas about mass and gravitational attraction seemed solid, everything in the observable universe (for the most part) was accounted for and all was good. But when the bubble burst what did physicists do? They go and invent an answer, but at least things work now, right?
No. The answer was not "invented". The evidence was collected and reviewed by many many people and the most likely explanations, the ones that required the least changes of physics were chosen. This is how science works.

Why would not it be more feasible to look at the fundamentals of physics instead of invent an idea that in large part cannot be observed or tested in a lab? If the problem involved mass and gravitional attraction and how galaxies worked, let's look at the fundamentals which were laid down hundreds of years ago, people. Could it be that we need an update on what we assume to be the fundamentals of physics?
Unlikely. Because the fundamentals work. And they work well. Exceedingly well. So well that to the limits of our capabilities of observing and measuring them they work exactly like we think they do. The technological leap of the latter half of the 20th century was a direct result of understanding these fundamentals. You want to rethink the fundamentals, but we cannot, because they are constantly being looked at, used, and investigated, and so far we have not found ANYTHING that tells us they are wrong. Not even dark matter or dark energy tells us the fundamentals are wrong.

Obviously, our fundamental ideas about mass and gravity are lacking. I'd be willing to bet a unified theory will leave dark matter and dark energy at the wayside.
They will not. What you don't realize is that dark matter and dark energy are "placeholder" names for effects we don't currently understand very well. If we observe a dark matter particle in a lab in the future it will most likely be re-classified and renamed. If it turns out that there is no such thing as matter that doesn't interact except through gravity, and it was simply a misunderstanding about gravity on the large scales, then so be it. Dark Matter is explained accordingly. Similarly dark energy will be looked into and if possible explained. The key is to understand that these effects will NOT go away. We observe them happening and nothing is going to change that. What we choose to name them is irrelevant.

Nothing about dark matter or dark energy is any different than any of science has been before. We see something, we give it a name, we try to explain it. If it's something new, great. If it's not, also great.
 
  • #10
10
0
The presence of dark matter halo is detected through gravitational lensening effect of a galaxy, whose luminous portion are robbed away by another galaxy . So the idea of dark matter becomes more prevailing than theories modifying the expression of newtonian gravity on large scales.
 
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  • #11
537
1
from what I've heard from professors: dark matter simply does a better job of explaining the galaxy mass problem than modified newtonian gravity does.

there's no reason to throw it out because it "sounds wacky" or something like that

throw it out based on observations. So far observations have not ruled out dark matter. Or so I have been told.
 

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