Archive for modified gravity

Cosmology Talks: Dan Thomas on the first model-independent cosmological simulations of modified gravity

Posted in The Universe and Stuff with tags , , , , , , , on April 7, 2021 by telescoper

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting. This one was published just yesterday.

In the talk Dan Thomas discusses his  recent work first creating a framework for describing modified gravity (i.e. extensions of general relativity) in a model-independent way on non-linear scales and then running N-body simulations in that framework. The framework involves finding a correspondence between large scale linear theory where everything is under control and small scale non-linear post-Newtonian dynamics. After a lot of care and rigour it boils down to a modified Poisson equation – on both large and small scales (in a particular gauge). The full generality of the modification to the Poisson equation allows, essentially, for a time and space dependent value for Newton’s constant. For most modified gravity models, the first level of deviation from general relativity can be parametrised in this way. This approach allows the method to use to  constrain modified gravity using observations without needing to run a new simulation for every step of a Monte Carlo parameter fit.

P. S. A couple of papers to go with this talk can be found here and here.

Modified Gravity: Evidence from Cavendish Experiments?

Posted in The Universe and Stuff with tags , , , , , on January 15, 2019 by telescoper

A paper by Norbert Klein caught my eye as I tried to catch up on my arXiv reading after a couple of days away last week. It’s called Evidence for Modified Newtonian Dynamics from Cavendish-type gravitational constant experiments and the abstract reads:

Recent experimental results for the gravitational constant G from Cavendish-type experiments were analysed in the framework of MOND (Modified Newtonian Dynamics). The basic assumption for the analysis is that MOND corrections apply only to the component of the gravitational field which leads to an accelerated motion of the pendulum body according to Newtons second law. The analysis is based on numerical solutions of the MOND corrected differential equation for a linear pendulum at small acceleration magnitudes of the order of Milgroms fundamental acceleration parameter a0 = 10-10m s-2 for the case of a mixed gravitational and electromagnetic pendulum restoring force. The results from the pendulum simulations were employed to fit experimental data from recent Cavendish-type experiments with reported discrepancies between G values determined by different measurement methods for a similar experimental setup, namely time of swing, angular acceleration feedback, electrostatic servo and static deflection methods. The analysis revealed that the reported discrepancies can be explained by MOND corrections with one single fit parameter. The MOND corrected results were found to be consistent with a value of G = 6.6742 x 10-11 m3 kg-1 s-2 within a standard deviation of 14 ppm.

I have edited the abstract slightly for formatting and added the link to an explanation of MOND. You can find a PDF of the paper here.

I blogged about the discrepancies between different determinations of Newton’s Gravitational Constant G a few years ago here, where you can find this figure:

The claim that Modified Newtonian Dynamics can resolve these `discrepancies’  is very bold and I’m very skeptical of the arguments presented in this paper. It seems to me far more likely that the divergence in experimental measurements is due to systematics.  If anyone else has different views, however,  please feel free to share them through the comments box.

Strong constraints on cosmological gravity from GW170817 and GRB 170817A

Posted in The Universe and Stuff with tags , , , , , on October 24, 2017 by telescoper

One of the many interesting scientific results to emerge from last week’s announcement of a gravitational wave source (GW170817) with an electromagnetic counterpart (GRB 170817A) is the fact that it provides constraints on departures from Einstein’s General Theory of Relativity. In particular the (lack of a) time delay between the arrival of the gravitational and electromagnetic signals can be used to rule out models that predict that gravitational waves and electromagnetic waves travel with different speeds. The fractional time delay associated with this source is constrained to be less than 10-17 which actually rules out many of the proposed alternatives to general relativity. Modifications of Einstein’s gravity have been proposed for a number of reasons, including the desire to explain the dynamics of the expanding Universe without the need for Dark Energy or Dark Matter (or other exotica), but many of these are now effectively dead.

Anyway, I bookmarked a nice paper about this last week while I was in India but forgot to post it then, so if you’re interested in reading more about this have a look at this arXiv paper by Baker et al., which has the following abstract:

The detection of an electromagnetic counterpart (GRB 170817A) to the gravitational wave signal (GW170817) from the merger of two neutron stars opens a completely new arena for testing theories of gravity. We show that this measurement allows us to place stringent constraints on general scalar-tensor and vector-tensor theories, while allowing us to place an independent bound on the graviton mass in bimetric theories of gravity. These constraints severely reduce the viable range of cosmological models that have been proposed as alternatives to general relativistic cosmology.

The Great Dark Energy Poll

Posted in The Universe and Stuff with tags , , on June 8, 2017 by telescoper

Yesterday was a very busy day: up early to check out of my hotel and head to the third day of the Euclid Consortium meeting for the morning session, then across to the Institute of Physics for a Diversity and Inclusion Panel meeting, then back to the Euclid Consortium meeting for the last session of the day, then introducing the two speakers at the evening event, then to Paddington for the 7.15 train back to Cardiff. I was not inconsiderably tired when I got home.

I had to bale out of the evening session to get the train I was booked on, but it seemed to be going well. Before I left, Ofer Lahav asked for an informal show of hands about a few possibilities relating to the nature of Dark Energy. Since today is polling day for the 2017 General Election, I thought it might be a good idea to distract people from politics for a bit by running a similar poll on here.

There are lots of possibilities for what dark energy may turn out to be, but I’ve decided to allow only six broad classes into which most candidate explanations can be grouped:

  1. The cosmological constant, originally introduced as a modification of the left hand side of Einstein’s general theory of relativity – the side that describes gravity – but more often regarded nowadays as a modification of the right-hand-side representing a vacuum energy. Whichever interpretation you make of this, its defining characteristic  is that it is constant.
  2.  Modified gravity,  in other words some modification of the left-hand-side of Einstein’s equations that manifests itself cosmologically which is more complicated than the cosmological constant.
  3. Dynamical dark energy, i.e. some other modification of the energy-momentum tensor on the right-hand side of Einstein’s equation that looks like some form of “stuff” that varies dynamically rather than being cosmologically constant.
  4.  Violation of the cosmological principle by the presence of large-scale inhomogeneities which result in significant departures from the usual Friedman-Robertson-Walker description within which the presence of dark energy is
  5. Observational error, by which I mean that there is no dark energy at all: its presence is inferred erroneously on the basis of flawed measurements, e.g. failure to account for systematics.
  6.  Some other explanation – this would include the possibility that the entire standard cosmological framework is wrong and we’re looking at the whole thing from the wrong point of view. If you choose this option you might want to comment through the box below what you have in mind.

Well, there are the six candidates. Make your choice:

A Plug for Some Research…

Posted in The Universe and Stuff with tags , , , , on May 12, 2014 by telescoper

Very busy today so I just thought I’d give a bit of publicity to a paper that’s just been accepted for publication. I’m actually one of the authors, but the other guys (Dipak Munshi of Sussex, Bin Hu of Leiden, Alessandro Renzi of Rome, and Alan Heavens of South Kensington Technical Imperial College) did all the work! I’m posting it mainly to remind myself that there is a world outside of administration. If it weren’t for my inestimable (STFC-funded) postdoc, Dipak Munshi, I don’t know where my research would be!

Here is the abstract:

We use the optimised skew-spectrum as well as the skew-spectra associated with the Minkowski Functionals (MFs) to test the possibility of using the cross-correlation of the Integrated Sachs-Wolfe effect (ISW) and lensing of the cosmic microwave background (CMB) radiation to detect deviations in the theory of gravity away from General Relativity (GR). We find that the although both statistics can put constraints on modified gravity, the optimised skew-spectra are especially sensitive to the parameter B0   that denotes the the Compton wavelength of the scalaron at the present epoch. We investigate three modified gravity theories, namely: the Post-Parametrised Friedmanian (PPF) formalism; the Hu-Sawicki (HS) model; and the Bertschinger-Zukin (BZ) formalism. Employing a likelihood analysis for an experimental setup similar to ESA’s Planck mission, we find that, assuming GR to be the correct model, we expect the constraints from the first two skew-spectra, S(0)   and S(1), to be the same: B0 <0.45  at 95  confidence level (CL), and B0 <0.67  at 99  CL in the BZ model. The third skew-spectrum does not give any meaningful constraint. We find that the optimal skew-spectrum provides much more powerful constraint, giving B0 <0.071  at 95  CL and B0 <0.15  at 99  CL, which is essentially identical to what can be achieved using the full bispectrum.

It’s part of a long sequence of papers emanating from work done by Dipak (with various combinations of co-authors, including myself) which have been aimed at optimising the use of statistical techniques for detecting and quantifying possible departures from the standard model of cosmology using various kinds of data; in this case the paper is entitled Probing Modified Gravity Theories with ISW and CMB Lensing; `ISW means the Integrated Sachs-Wolfe Effect and CMB is the cosmic microwave background. This kind of work doesn’t have the glamour of some cosmological research – I don’t think we’ll be writing a press release when it gets published! – but it is the kind of preparatory analysis that is essential if cosmologists are to make the most of present and forthcoming observational data, which is why we keep plugging away…