# Gravity and developments in GR after WMAP 5 year data

## What will Gravity turn out to be? (Please read extended descriptions before voting!)

7 vote(s)
19.4%

3 vote(s)
8.3%

15 vote(s)
41.7%

5 vote(s)
13.9%

6 vote(s)
16.7%
1. Apr 2, 2008

### Wallace

Since the WMAP 5 year data is now out and contains no nasty new surprises, what does everyone think about the current and future status of gravity? Please read these more detailed descriptions of the poll options before you vote! Note that these options are all basically beyond what we can know with any kind of confidence at present, but what is your gut feeling? It might be interesting to compare the poll results to what we know in 5 years (or more) time!

1) GR is the true theory of gravity and is correct for every situation where it gives a sensible results (i.e. away from singularities). Any future quantum gravity theory might explain in more detail the nature of the centre of black holes and the very very very early Universe but will leave all other GR predictions unchanged. In this case dark matter is real and dark energy is either really some energy source or a true cosmological constant.

2) As above but dark matter and/or dark energy will be shown to be a result of spurious errors either in observation (i.e. for some reason distant SN are intrinsically dimmer for some reason other than acceleration) or a result of the FRW model for GR being a poor approximation and an improved model, using GR only, explains the data without dark energy and/or dark matter.

3) GR turns out to be a poor model for gravity on large scales and a new theory is developed that explains away dark matter and/or dark energy as simply the result of wrongly interpreting the data with GR as the gravity model. This new theory may or may not be a full quantum gravity theory that also explains the centres of black holes and the very very very early Universe without producing singularities.

4) The pessimistic option. We are inherently constrained in terms of what we can ever be able to measure from Earth and therefore we will never be truly able to know whether GR+dark stuff or a new theory of gravity is a better option. Both may be able to explain the data to some degree and we won't be able to say for sure which one is correct.

5) Fence sitter option. At this stage it is impossible to have any gut feeling about the eventual outcome, all or a few of the above options are equally likely.

Note that none of the options mention any direct laboratory detection of dark matter and/or dark energy. If this was to occur that information would feed back into cosmology to support some and reject other options, so if you think this will or won't occur that can guide your choice as well.

Lets not get overly speculative, but what is your leaning at the moment?

2. Apr 2, 2008

### MeJennifer

My vote

GR is incorrect if it needs any kind of fudge factor such as the cosmological constant.

3. Apr 2, 2008

### marcus

You know what I'm going to say. Classic GR has to be wrong because it has singularities (and other reasons, like one Jenny suggested.)
GR will inevitably be replaced by a quantized version. It is a good bet that the renovation will affect predictions at large scale and low curvature---as collateral byproduct to resolving singularities.

There will be a new understanding of space time and matter. That will bring about a new perspective on things that are rather ad hoc at the moment, like inflation, acceleration etc. When we have a better understanding of geometry and matter---the fundamental DOF they emerge from---we will be able to say more clearly what DM and DE are. When we can say what they are, it will be a better time to ask whether they "really exist."

As observed effects that need to be explained, they exist (is my feeling). I don't buy Wiltshire-ian dismissal, as just some happenstance unevenness. There is something that needs deeper physics to explain. But the customary way of explaining things by developing some new particle field might not be the requisite new physics this time.
That way of explaining stuff may be getting a bit old.

4. Apr 2, 2008

### Wallace

Edit: As far as I can see there haven't been votes from either of you guys registered? Is this a bug in the poll or do none of the options suit?

The cosmological constant arises quite naturally in the derivation of the Einstein Field Equations, it has to be manually set to zero in order for it to not appear. It is also a very specific quantity that makes very specific predictions, not infinitely tuneable in the way that say a more generic dark energy term might arguably be. I think it's a bit much to suggest that it is merely a fudge factor. GR with a cosmological constant as a geometric effect (i.e. 'just how gravity is') is a perfectly reasonable and readily falsifiable model of gravity.

Thanks also for your input marcus.

Last edited: Apr 2, 2008
5. Apr 2, 2008

### MeJennifer

You mentioned this before and you failed to convince me how it arises naturally in the derivation of the Einstein Field Equations.

For those interested here is Wallace's explanation why he thinks it arises naturally, or in this case he claims it is actually suggested:

Sorry Wallace but I see nothing in your explanation how, where, or why it would arise naturally or is in any way suggested.

6. Apr 2, 2008

### Wallace

I'll leave what I posted previously (as linked to above) as my explanation for now. I'll leave it to another day (and thread) to say anything more detailed. Lets not get too OT in this thread by discussing this point at length. If you consider that GR with a CC is not really GR but actually a different theory of gravity (containing an additional fudge factor) it looks like option 3 would be more to your taste perhaps? That's of course if the CC turns out to be the correct fudge!

Last edited: Apr 2, 2008
7. Apr 2, 2008

### my_wan

I was forced to vote "GR is correct but DE and/or DM are not real". Sure it merely a bet and not a statement of empirical knowledge but it's the one I arrived at very carefully.

Wrt GR being "correct" requires some qualifiers. The "extended descriptions" didn't allow a really good match to what I would have liked to choose. I think GR as presently interpreted fails to account for the full range or interactions of symmetries involves. Like the Cosmological Constant debate, issues remain in which the assumptions are not fully defined. This does not mean GR is wrong in the sense that that it must be replaced, only constrained properly. It might, however, be that an equivalent but mathematically different version is needed to help explore the assumptions. In fact such an equivalence is required if GR and QM are to be brought under the same framework.

ETA: Oh yeah. I find the idea that observations will never decide too alien a concept for my taste.

Last edited: Apr 2, 2008
8. Apr 2, 2008

### MeJennifer

A theory to me needs to be able to predict things. Plugging in constants to fit current observations is not predicting anything. Every time we discover something new the constants are adjusted accordingly. Now that should give some a clue as what they are actually doing instead of creating a theory wouldn't you agree? Dark energy, dark matter, cosmological constants, dynamic parameters, perhaps to some these things sounds very impressive, but to me it is similar to medieval angels, invisible pink unicorns or tiny green turtles. Old wine in new bags!

Last edited: Apr 2, 2008
9. Apr 2, 2008

### sysreset

I just voted. I think GR is wrong, but only insofar as it breaks down at the the extremes of event horizons and it also breaks down for extremely large scales. It is still remarkable how well it predicts observations between these extremes over so many magnitudes of order, just as it is remarkable how well Newtonian gravity makes predictions within its scope. For me, both DM and DE are fudge factors for a failing theory rather than true derivatives of observational science. In a sense, finally discovering the limits of GR is a triumph of science and a testament to our abilities to push the boundaries of our observations so far outward. The theory that will eventually supplant GR may or may not be developed in our lifetimes, but given enough time and patient observation, it will be developed by humankind.

10. Apr 2, 2008

### Wallace

Are Maxwells equations like medieval angels since they need you to fit $$\epsilon_0$$ and $$\mu_0$$ to data? Likewise what about particle physics? There are more free parameters set by data alone in the standard model than you can poke a Feynman diagram at! What about that charlatan Newton, sure gravity goes as 1/r^2 but you can fit that to any old G that you couldn't predict before you do the measurement.

I agree, as I stated in previous posts, that allowing dark energy to have arbitrary properties gives it too much freedom, the point about the CC is that a model with this non-zero has a very particular form that does not have infinite freedom. Sure you don't know the value of one parameter, but changing this value, like changing G in Newtonian gravity doesn't give you infinite freedom. The data could very easily suggest that there is no value of the CC that fits, in the same way as say non-keplarian orbits would tell you that Newtonian gravity didn't work, regardless of the value of G.

To turn the tables though, what do you think people should be doing if the current method doesn't suit your taste? Even if dark energy is a complete furphy, by measuring carefully what its properties are that gives us a way of frameworking new data. If a new theory comes that says 'if you you thought dark energy existed, it would look like this' and that matches the measurements then the new theory looks a goer. If you hadn't gone after the dark energy properties in the first place though, you'd never have progressed the theory.

There were plenty of very good astronomers carefully calculating the periods of epicycles that allowed Copernicus to realise a better model. Without the data, which was taken with the motivation of a different model, we would never have improved the theory.

11. Apr 2, 2008

### rbj

(doesn't matter)

Jennifer, i thought you dropped off of the edge of our flat world!

it's good to see your presence again.

bestest,

12. Apr 3, 2008

### Garth

I really do not have a problem with singularities in GR, we may not like the idea but that doesn't make their existence impossible.

The standard $\Lambda$CBM theory fits the WMAP etc CMB data very well. I make just one comment; the theory has at least six free parameters that are adjusted to fit the data. If you have enough free parameters you can make anything fit. The evaluation of these parameters is theory dependent, change the theory and their evaluations change as well. A modified theory with a different set of parameters might also be made to fit.

On the other hand there is the question of a quantum gravity theory. When such is discovered and tested I cannot imagine that GR will be not be modified in the non-quantum regime in some way.

Anomalies such as the Pioneer Anomaly and Flyby Anomalies may already be indicating our present understanding of gravity needs such modification.

Also there may be an age problem at high z, where there seem to be some evolved structures......
This may indicate that we do not understand the early expansion history of the universe very well, possibly DE acts in the BBN epoch thus elongating the time of nuclearsynthesis. This would lift the restriction on baryonic density.

Once the hypothetical non-baryonic particle is discovered in the laboratory, its properties measured and found to be concordant with cosmological constraints then we will know what we are talking about. Until then an open mind must be kept.

There seems to be evidence of the existence of something out there, MOND type theories keep getting more complicated to fit new data, which is the sure sign of an epicycle. However the same can be said for the old FWL cosmology with is invocation of Inflation, DM and now DE, all at present undiscovered in the laboratory.

DE is more difficult to pin down. I agree with Wallace that it could be the cosmological constant. Note in this case there is no actual DE, no quintessence or whatever, it is just the way gravity behaves, attractive at short ranges and repulsive at cosmological distances. And yes, MeJennifer, GR is such a woolly theory that it cannot predict what value the CC ought to take! It was this inadequacy of the theory that led relativists over many decades to set the constant at zero to save our embarrassment.

Unfortunately you cannot distinguish observationally between the CC and false vacuum energy, which has the same effective equation of state. The problem with false vacuum energy density of course is that quantum expectations are ~~10120 times too large!

So my vote? I do not have an option in the poll.

I say GR will be modified by a future quantum gravity, the local predictions of which would also explain DM and DE at galactic and cosmological ranges.

Just my two pennyworth....

Garth

Last edited: Apr 3, 2008
13. Apr 3, 2008

### MeJennifer

In the case you might misunderstand me Wallace, I think that cosmologists do excellent, admirable and useful work in the measurements they take, interpreting them, and as you wrote, frameworking them. But do we see any theory development? Don't instead we see GR + various fitting mechanisms presented as new theories?

Last edited: Apr 3, 2008
14. Apr 3, 2008

### Wallace

It could just be a case of different meaning of the term 'theory'. There are many different physical theories that suggest what dark energy might be (far too many to list!), so instead of treating them all individually, dark energy is usually parametrised by some convenient simple function (this is probably not news to you I guess), a common form being to parametrise the DE equation of state as $$w(a) = w_0 + (1-a)w_a$$. This is certainly not a 'theory' of dark energy, just a convenient way to get a handle on the basic properties of it. I don't think anyone presents such a thing as a theory as such.

I think the reason that we hear so much about the parametrisations rather than actual micro physical theories (even though plenty of them get suggested in the literature) is simply because there are too many theories that look far too similar. By fitting generic w(a) parametrisations however, we can rule out entire classes of theories, and then examine in more detail those that are left. At least that's how the community is thinking things will turn out in the next few years, but it remains to be seen what the data says and no doubt there may be some surprises.

So I think the problem is not too little theory development but too much, far too much for anyone to be able to consider every new idea individually, hence the focus on parameter fitting, even if those parametrisations are admittedly not actual physical theories.

15. Apr 3, 2008

### MeJennifer

Good point.

16. Apr 3, 2008

### CosmiCarl

Fudging

I just voted, and I mostly agree with MeJennifer's words, above. But I do not agree when she says a theory with ANY Fudge Factor is bogus. It is the MAGNITUDE of the fudging that bothers me.

Certainly, GR successfully predicted Gravitational Lensing and Time Dilation, BUT, when observations to date suggest ~70% of the universe is stuff we do not understand at all, I feel very uncomfortable calling the theory "right."

The Methodological limitations that led to DM and DE are what bother me the most of all.

I also agree with Garth: I see no fundamental problem with a theory having Singularities, per se. At least there is an explicit recognition of instances where the theory does not apply.

And the Pioneer Anomaly seems to me to be a VERY important clue. Has anyone explored how that percentage "error" compares with the numerical percentages that observation deviates from theory in other cases?

17. Apr 3, 2008

### Jenk

I just arrived, hello, voted that GR is wrong, and will be replaced by a new theory sooner or later, without DM and/or DE.

I think it'll turn out that DM is real, DE unreal. An interesting question is: if DM was discovered, and it fitted very well with some of our theories but not others, would we abandon the theories it didn't fit with? This is a barometer for whether an idea is a fix up factor or not.

What if DM was discovered but it contradicts big bang theory - would people accept it? Well... it happened in 2002, and was widely ignored. Two Dutch astronomers found HII in an edge-on galaxy, it was difficult to detect, but they did it. it was in the right amount, I think about 10 times the visible mass. 5 - 15 times was needed. But that goes against big bang theory, so out of the window went their excellent work - for many people anyway. I guess that means DM is in some ways a convenient tool only.

18. Apr 3, 2008

### Wallace

By definition if something is detected in a telescope it is visible!
Dark Matter cannot be HII since if would have seen it long ago in many places in the Universe and we would never have arrived at the idea of dark matter in the first place.

Last edited: Apr 3, 2008
19. Apr 3, 2008

### Jenk

HII is invisible and very difficult to detect. It's dark matter, but it doesn't fit with standard big bang theory

20. Apr 3, 2008

### Wallace

HII is indeed difficult to detect in the quantities that it occurs in the Universe. If however, the enormous amount of extra mass needed that lead to the postulation of dark matter was in fact HII, then in such vast quantities and concentrations it would have observable effects that we simply don't see.

In any case, HII by definition is warm (well pretty hot actually) whereas dark matter must be cold. That is, if there really is stuff out there that really explains the data by being a massive and dark then it must be cold. HII is far too warm to be dark matter in the sense of what is needed to drive structure formation. If you make dark matter too warm it washes out structure and you wouldn't get the agreement in the data that we see for the cold dark matter model.