If we celebrate provocative new experimental findings, we should also celebrate the careful null results (experiments that agree with existing theories) on which much of science is based. Back in October we pointed to a new analysis that used observations of gravitational lensing by large-scale structure to test Einstein’s general relativity on cosmological scales, with the intriguing result that it didn’t seem to fit. And the caveat that it probably would end up fitting once we understood things better, but it’s always important to follow up on these kinds of clues.
So now we understand things a bit better, and a number of people have been working to dig into this apparent anomaly. Here is a new paper from this week, that presents their own way of using these kinds of data to test GR against large-scale structure.
Testing General Relativity with Current Cosmological Data
Authors: Scott F. Daniel, Eric V. Linder, Tristan L. Smith, Robert R. Caldwell, Asantha Cooray, Alexie Leauthaud, Lucas LombriserAbstract: Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.
One issue, as we noted way back when, is that it’s very hard to “test GR” without committing yourself to a model of the mass and energy sources that are causing the curvature of spacetime. So the game is to make some plausible assumptions and see where you go from there. This group seems to have assembled a sensible framework for testing deviations from Einstein, and come back with the answer that everything is on the right track.
We keep getting new and better data, of course, so we’ll keep testing. I suspect Einstein will continue to be right, but probably a lot of people thought Newton would continue to be right a century ago.
Now if we could only detect gravitational waves …
“We should also celebrate the careful null results ”
I agree. Sometimes I hear talk like: “I didn’t discover anything that wasn’t expected so I don’t know if anyone is going to care about/cite my paper.”
However, often these projects were originally done because technically we weren’t sure and so finding everything fits theory is still an important find. (Although, it still may not be as highly cited as if something bizarre was found.)
I’ve heard of many biochemistry professors wanting to start a journal of negative results just for publishing these kinds to null results where things worked as expected.
I am not familiar with the physics or cosmology communities, but in biochemistry getting published (or funded) often depends on finding “new” results (or at least “me-too” results) and so you rarely take note of articles that support an existing established view.
Newton wasn’t right? His explanations are certainly insufficient for the current level of physics, but many of us suspect Einstein’s explanations will suffer a similar fate if physics continues to move forward. As Einstein put it,
” “Let ‘S’ represent the state of scientific knowledge at time ‘t’, and let ‘Q’ represent the correlative family of questions that can be posed at this juncture. Then it is perfectly possible that some future state, ‘U’, will be attained where all the questions ‘Q’ will be resolved, while yet there are then, at this time ‘t2’, further questions ‘Q2’ which are unresolvable at ‘t’–questions that are not members of ‘Q’ because they could not have been envisaged from the cognitive posture of ‘S’. This is a condition of affairs that can in principle, continue to exist ad infinitum.”
Kinda mean to call a man wrong when he existed at time ‘t’, and you are dealing with ‘Q2’ questions.
As a non-physicist I have a question about the relationship of GR to the LSS. Over the past year or so I have been looking at David L. Wiltshire’s papers on his Fractal-Bubble now Timescape hypothesis.
As I understand it his “radical conservative” approach to GR where he agues that dark energy is an artefact that derives from the assumption of homogeneity in the FRLW metric. Consequently as we live in a Universe where LSS makes the universe inhomogeneous, then FRLW is no longer fully applicable unlike it was up to reionization. He proposes when the LSS is taken into account then there is no need for for lambda and a non zero vacuum energy.
I do not have the grounding in GR to really follow his papers. So what do you guys think is it a plausible hypothesis? Is there no dark energy?
I guess we will have to wait until the observational tests give some sort of support to either the timescape hypothesis of the LambdaCDM model.
Sean, in the book you talked about GR being time symmetric and offered white holes as the time symmetric counterpart of black holes (if I understood correctly?)
I fail to see how GR is time symmetric. It says that massive objects always pull the “future” direction of light cones towards them, never the “past” part of the light cone. And, I’m not sure I understand how a black hole is reversible at all… As everybody keeps saying, once it’s in, it’s gone forever.. That explicitly needs the arrow of time.
More than 3 months ago there was another paper from a large weak lensing study with the updated COSMOS data, which among other things
showed that everything was all right with GR. It is beyond me why this study
was not cited by Daniel et al.:
“Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS”,
T. Schrabback et al., arXiv:0911.0053
> We keep getting new and better data, of course, so we’ll keep testing. I suspect Einstein will continue to be right, but probably a lot of people thought Newton would continue to be right a century ago.
What is it that Newton was wrong about? I was under the impression that Newton’s laws continue just fine, and that later physics merely *add* to the explanation.
That is, Newton’s description (his set of laws) was not wrong, merely incomplete. The description continues to match now whatever it was Newton designed it to match.
No?
Hi Sean,
Are there any measured deviations from General Relativity?
(I mean any experiments at all, not just the one you mention.)
Cheers
Mat
Just a few thoughts,
Conceptually, SR/GR is not in the same ball park with Newton…the concepts are very different. The predictions of SR/GR are very precise…there is no room to fudge.
If there were any measured and verifiable experimental deviations from SR/GR the whole concept would immediately be called into question- to date, it hasn’t been.
When careful measurements are being made, SR/GR affects experimental results even at very slow speeds relative to the speed of light. Near the speed of light, SR/GR and Newton bear not the sligtest resemblence.
If Satellite GPS did not correct for relativity- even at very slow (orbital) speeds- derived locations would be city blocks off…it would make GPS almost unusable. GPS with GR correction can land an aircraft 0/0 (BLIND) with perfect accuracy…even though landing an aircraft 0/0 is illegal…what if there is a cow on the runway?!
All that being said, within the Planck realm GR does not work (there are prople who have tried to make it work!)…it is fine- down to 10 to the minus 33rd Cm. GR has been experimentally verified to more than a dozen decimal places and there is no conceptual reason to believe it will fail until we try to measure to more than 30 places or so.
One important concept of Newton is still VERY much alive…the comncept of instantaneously or near instantaneously propagating gravity. From our frame of reference, gravity seems to propagate at the speed of light…in fact when we try to measure the “speed” of gravity, we measure it to travel at that speed. However, the universe would not stay together as we observe it if gravity did not propagate in a “Newtonian” fashion.
Instantly or near instantly propagating gravity relates to the fact that SR/GR does not work below 10 to the minus 33rd Cm. Singular and photonic characteristics as measured using the Einsteinian concept involve 0 or near zero/ infinities or near infinities…the latter two if the mass of the universe is finite, which it also has to be, in order for the universe to have evolved as it has according to the Standard Model.
Science is based on observation, in fact existence and consciousness are observation related. These realities make SR/GR very important. However, cosmologically we need to remember we live in a quantum universe, which we happen to observe in a certain, very specific and measurable way.
Since the entire universe is a quantum entity, we are forced to conclude that macroscopic or microscopic, cosmologically the universe is a very different place than what we observe and measure. This means we need to look for a concept of quantum gravity…we are forced to relate the universe of SR/GR which we live in and experence to the near 0 and near infnite realm of quantum fluctuations and black holes.
Actually, SR/GR implies just as precisely about its limits as about what it CAN do- as it makes testable predictions within the universe of space and time we live in.
Strictly speaking the GR concept involves a geometric point at the center of its curved geometry, so in that sense, GR is almost certainly wrong. We KNOW the Planck Realm exists at a certain level of scale and we measure the universe to also be a quantum entity.
A complete understanding of our quantum universe might “overthrow” SR/GR because Quantum Mechanics is just as conceptually different from SR/GR as SR/GR is from Newton. However, apples still “fall” on our head whether we use a newtonian conceptual framework, a relativisitic conceptual framework or (once we have devised something!) a quantum gravitational conceptual framework.
The idea of course, is to REALLY understand what is going on in this-here universe of ours!
Is the validity of General Relativity dependent on some test that supports the reality of dark matter and dark energy? If this is so, I would think that scientist who report on a test supporting the validity of a GR would make some remark about how their results are dependent on finding direct or indirect evidence of dark matter or more clarification of the nature of dark energy. Maybe this caveat was voiced in their sentence placed at the end of their article.
“These will need to be addressed to have confidence in claims of any detected deviation, as will allowance for expansion histories different from ‘LCDM’. (arXiv:1002.1962v1, p9)
Those “loose ends” as Sean calls them can be a real can of worms…when we “pull them”, our whole conceptual framework can come into question!
Sean’s book gets more interesting- and profound- as he continues. I hope nobody puts it down half-read! Anyone who does is definitely NOT doing himself or herself a favor.
Just to pick one little thing…
Sean was speaking of 5 dimensions with gravity being projected as 4 dimensions with no gravity at all. That reminded me immediately of the static or quasi-static universe (obviously the 5 dimensional scenario does have gravity, and it is the “parent” of the static 4D scenario).
4D with no time, no motion and change- and no gravity! Usually when scientists refer to the 5th dimension, they are really talking about adding an extra 3-space to the geometry. I only know the little about this study I got from Sean’s book, but what he says is very congruent with what I have learned. Einstein’s universe may be Einstein’s universe, yet its proper and correct description could well, in fact probably will go far beyond his description.
We see black holes and the singular condition as representing a certain relationship between the manifold and energy density…from our frame. However, is our frame cosmologically accurate? I think the chance our frame is cosmologically accurate is highly unlikely and we can see the “handwriting on the wall” in Quantum Mechanics.
Our reality as a projection of a higher dimensional order is dynamite.
I always liked the “Holodeck” on Star Trek. In one episode, the crew was interacting with the programmed people, and made friends with one person who was convinced he was absolutely “real”. The crew of the Enterprise told him he was NOT real, only a projection…a part of a program.
To prove he was only a projection, the crew opened the door to the outside, but warned him that if he walked out with them, he would simply disappear. The man, realizing the true nature of his existence, almost had a nervous breakdown and begged the crew to spare him. The crew assured him that his program was permanently stored in their archives and that from time to time he would be allowed to “exist”.
This is high quality science fiction, done by people with a good knowledge of modern science, and the scenario may be an omen of what we may indeed find out about the nature of reality.
Black Holes as layered information can be easily conceived if we realize that our particulate existence occurs as we electromagnetically observe what is “happening” on sub-microscopic event horizons. Come to think if it, the fact that we observe electromagnetically, relates far better to Quantum Mechanics than SR/GR!…
Are we living inside a black hole? What’s wrong with this logic: Early in the history of the universe its radius was much smaller than the Schwarzschild radius corresponding to the mass of the universe. An object smaller than its Schwarzschild radius is called a black hole.
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