
Quest to settle riddle over Einstein's theory may soon be over - dnetesn
https://phys.org/news/2017-02-quest-riddle-einstein-theory.html
======
nickhalfasleep
Imagine if scientists discovered we were existing in a simulated universe, and
that sumulation had an error in it's floating point library.

~~~
edblarney
It wouldn't be an error, it would just be an artifact of that Universe.

~~~
Nomentatus
I think that's pretty much what Intel said at first, way back when, but they
had to replace all those CPUs anyway.

------
raattgift
The preprint is here:

[https://arxiv.org/abs/1602.07670](https://arxiv.org/abs/1602.07670)

------
amelius
What are other implications of the speed of gravity being unequal to the speed
of light?

~~~
raattgift
I'll try to answer that via a couple of concrete examples of theories that
deliberately set the speed of gravity and light at different values for good
reasons.

The short answer is that General Relativity (GR) predicts that the speed of
light is equal to the speed of gravitational waves. In order to have a
difference between the speed, one needs a different theory from GR.

Moreover, the equality of the speed is well-tested at solar system scales;
there is good evidence that at least within a few billion light years from
here the speeds are the same; and there is good evidence that for at least the
age of the solar system (several billion years) the speeds have been the same.
A physically realistic theory has to match these lines of evidence.

Around General Relativity we've built the Standard Cosmology, and in that we
construct a preferred frame of reference -- the comoving frame, or the
cosmological frame -- in which we calculate the evolution of the size of the
universe under the influence of attractive and repulsive matter. The comoving
frame is the one containing all observers who see the universe as spatially
flat, homogeneous, isotropic, and have a view pastwards to a hotter denser
universe.

Our understanding of the universe decreases when the universe is very hot and
dense, and our view that far pastwards with current equipment is obscured. How
the early universe "behind the veil" of the oldest visible surface becomes
that particular surface raises some tricky questions about the behaviour of
the matter and spacetime in the very early universe.

Additionally we do not now have a microscopic description of all the
attractive matter in the standard cosmology, and recent evidence suggesting an
acceleration of the metric expansion raises questions about whether our
microscopic description of repulisve matter is correct. Moreover, our
macroscopic dsecsriptions of these still need some "help" in the early
universe in order to produce homogeneity, isotropy and spatial flatness given
the visible attractive matter and also the invisible bits of the Standard
Model of particle physics, in the form of another form of repulsive matter.

From time to time it becomes fashionable or interesting to explore
modifications to General Relativity that would eliminate the need for one or
both types of repulsive matter. This is a trendy moment.

In General Relativity there is a universal coupling to the metric tensor.
Gravitational waves are values propagating in this tensor field, and it is
common to talk about them as propagating at the "tensor speed of sound".

One popular approach to modifying GR is to introduce a square scalar component
that multiplies with the metric tensor to produce the gravitational action.
Such theories are called scalar-tensor theories of gravity.

In scalar-tensor gravity you can make the tensor speed of sound unequal to the
speed of light in a variety of ways.

One approach is to make the scalar value location-dependent, with its
evolution modifying the gravitational attraction between clumps of matter. In
this sort of approach, one relates the scalar and tensor values to each other
in such a way that one would still talk about just the "tensor speed of
sound", with the scalar driving changes in that value (typically making the
tensor sound speed slower in the past).

The phys.org describes a paper that deals with a subset of this type approach,
which I'll get to further below.

Another approach is to have two metrics that do not couple universally. For
instance, one could retain the metric tensor from General Relativity which
couples to massive matter (like electrons and neutrinos) and add a metric
scalar-tensor which couples to massless matter (like light and gluons). This
lets the tensor speed of gravitational waves produced by light (and other
massless particles) be slower than light.

Here's a concrete example of this approach:

Afshordi & Magueijo [
[https://arxiv.org/abs/1603.03312](https://arxiv.org/abs/1603.03312) \- I'll
call this A&M] are exploring a (scalar-tensor) bimetric theory where, in the
hot, dense, early universe the two metrics are unequal; massive matter couples
to one metric, and (after electroweak symmetry breaking) while massless matter
(mainly meaning light, since gluons stay confined in hadrons) couples to the
other. The effect is that the sources of the former metric can move very
slowly compared to light and the metric that it sources; it is just as valid
to say that light moves much faster than, for example, the lightest massive
particles. The result is that in the early universe, light can carry momentum-
energy from one region of the early universe to another more efficiently than
it can in the present day universe. Additionally, light zips ahead of its own
gravity (or equivalently sheds cerenkov-like (tensor) gravitational radiation)
and so even at the highest energies and energy-densities, light can homogenize
the temperature of the early universe at extremely long length scales without
disrupting structure formation and in particular without significantly
changing the structure of the Cosmic Microwave Background.

After this temperature-evening process solves the horizon problem there is a
phase change that causes the massless-matter metric to "freeze" into a
configuration that is exactly identical to the massive-matter metric.

This leads to a present-day universe that reproduces all the observational and
experimental successes of General Relativity and an early universe that the
authors claim solves the horizon problem without cosmic inflation.

(They refer to other work on similar bimetric models that suggest that one or
more other problems that cosmic inflation was introduced to solve --
homogeneity, isotropy and flatness -- might also be solved by their model).

Now onto the single scalar-tensor metric approach:

The Lombriser & Lima paper [
[https://arxiv.org/abs/1602.07670](https://arxiv.org/abs/1602.07670) \-- I'll
call this L&L ] that's the subject of the phys.org article deals with a subset
of Horndeski gravity theories. Horndeski theory are a general family of
scalar-tensor theories that become General Relativity when the scalar freezes.
It is very general and the scalar can relate among other things to a f(R)
gravity -- a function on the Ricci scalar R -- or a f(\hat{G}) gravity -- a
"variable G" theory, where G is Newton's constant. L&L focus on Horndeski
theories that seek to eliminate the need for a cosmological constant term (or
similar dark energy source term in the cosmological frame).

(Aside: A&M somewhat distinguish themselves from Horndeski gravities starting
at the top of their p. 2. They do not use the vocabulary of Horndeski theory
or expand on their argument, and additionally the A&M paper itself is focused
on cosmic inflation rather than dark energy. So the applicability of the L&L
argument is not especially clear.)

L&L argue that generically in a Horndeski theory that is able to reproduce the
metric expansion of space, the evolving scalar induces a damping of (tensor)
gravitational waves different from that of General Relativity; in particular,
in order to get rid of dark energy you have to slow down tensor waves in the
earlier universe, and speed them up in the later universe. This lets galaxy
clusters separate from each other faster in the later universe than they do in
the earlier universe. But this approach also produces observables in
electromagnetically bright events that also produce detectable gravitational
waves (e.g. inspiralling neutron stars that explode into a GRB or type II
supernova and possibly shedding gravitational waves from fallback accretion
onto the remnant(s)). Moreover, such events for a wide range of theories are
within the threshold of forthcoming gravitational wave observatories.

Additionally in these theories, as light would zip along faster than the
tensor speed of sound, they would shed Cerenkov-like (tensor) gravitational
waves, which may have [a] their own observables but [b] reduce the speed of
light relative to the speed of gravity when the path the light takes through
spacetime is long. The authors argue that studies of objects with known
spectra at different high redshifts should directly test this aspect of dark-
energy-free Horndeski gravities.

Finally, the L&L paper doesn't look too deeply at the semiclassical picture,
asking about whether other massless particles -- gluons, namely -- racing
ahead of gravity should produce observables. Neither does A&M's paper. And
both papers stay far away from questions of quantum gravity.

~~~
rosstex
Wow, thank you for this excellent heap of info!

I noticed you reference the Cosmic Microwave Background in a nuanced way, "How
the early universe "behind the veil" of the oldest visible surface becomes
that particular surface" before mentioning the name itself. You never quite
define the CMB or highlight its significance, which may be a bit confusing for
readers. Otherwise, awesome stuff!

~~~
raattgift
Well, we might measure other BB relics like the cosmic neutrino background or
stochastic gravitational waves, someday. :-)

> You never quite define the CMB or highlight its significance

I'm not sure how significant it is (as a veil).

If you take an initial values approach, you can put the IV surface pretty much
anywhere in spacetime and let known physical laws do their thing. The problem
is that we have no way of knowing (with current telescope technology) if we
are choosing reasonable initial values on a surface much earlier the surface
of last scattering, except if it mechanically produces something a lot like
our view of the universe at the correct time. That's about as anthropic as it
gets.

Likewise, for purely theoretical reasons we end up on shaky ground when
marching into the very distant past from initial values on a reasonable
spacelike hypersurface set nearer the present day (what happens to matter in
strong gravity? are all physical laws really time-reversible? and similar open
questions), and unfortunately with present telescope technology we can't see
past the surface of last scattering to where this shakiness would manifest
distinguishing observables.

Having things other than photons to look at will help (and so will better
views of the CMB), and is likely to alter when exactly one would put "the
veil".

I tend to doubt the cosmological significance of things that almost certainly
can be made to move or vanish when we puny humans invent and launch new
observatories.

