## Is cosmology completely broken?

Thirty years ago, when I was taught cosmology as an undergraduate, it felt pretty much like a subject that was close to being fully described: indeed this was the era when Stephen Hawking could announce that we were close to a “theory of everything”.

In simplified form the cosmology was this: the universe (and there was just one) was created 13 billion years ago in a “Big Bang”, the echo of which we could see in the cosmic microwave background (the red shift of which allowed us to place an age of the universe itself). Since then the universe had been expanding and the key question was whether there was sufficient mass to halt this expansion (i.e. that gravitational attraction would overcome the impulse of the Bang) or would it expand for ever. Contemporary observations suggested that the universe’s mass was close to the critical value that separated these two outcomes and the big issue seemed to be getting better observations to determine this question. Beyond that, cosmology was not very open…

Core to the cosmology we were taught was a very simple yet extremely powerful idea: the cosmological principle. Namely, that at a sufficiently large scale and at the same point in time, the universe looks the same in every direction when seen from any point. In fact this idea was treated as close to axiomatic.

(Of course, without some form of cosmological principle then cosmology itself becomes pretty metaphysical – if our observations and experiments have no general validity they cannot tell us much about the universe.)

Three decades later, though, and cosmology is something of a mess. Our observations not only suggest that visible matter is a minority of all matter, but that matter (including the unseen and so far undetected “dark matter”) is a minority of the matter-energy (the two being equivalent as $E=mc^2$ famously tells us) and that a “dark energy” dominates and is actually accelerating the universe’s expansion.

Dark energy is little more than a term in a mathematical equation, something that reminds me all too much of phlogiston but it’s fair to say that most cosmologists are satisfied that it exists.

But not all of them.

As an excellent and highly accessible article in the New Scientist  makes clear, a number are arguing that the problem is that the cosmological principle, or at least our rigid application of it to our observations, is leading us astray. For if the universe was fundamentally lumpy and not smooth at a large scale then it could create the “illusion” of dark energy: put simply if some bits of the universe had less matter in them, then they would expand faster (or in general relativistic terms have greater negative curvature) as gravity would not hold them back – but if we did not factor for that in our interpretations of the observations we would instead assume it was a general effect that applied everywhere.

The advocates of standard cosmology do not deny that the curvature of the universe impacts the passage of the light we see when we observe it – but respond that the homogeneity of the universe at a large scale – i.e., the cosmological principle, means that the patches of negative curvature are cancelled out by the patches of positive curvature and the overall impact on our observations is neutral.

The impact of clumpyness/emptyness on our observations is called “backreaction” and key question for the black energy sceptics is whether it leaves traces in observations that we misinterpret as pointers to dark energy.

The debate, as so often in scientific research is quite brutal – if you say someone’s conclusion is “unphysical” it is pretty much like accusing them of being no good at their job…

The abstract of the paper Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?:

No. In a number of papers Green and Wald argue that the standard FLRW model approximates our Universe extremely well on all scales, except close to strong field astrophysical objects. In particular, they argue that the effect of inhomogeneities on average properties of the Universe (backreaction) is irrelevant. We show that this latter claim is not valid. Specifically, we demonstrate, referring to their recent review paper, that (i) their two-dimensional example used to illustrate the fitting problem differs from the actual problem in important respects, and it assumes what is to be proven; (ii) the proof of the trace-free property of backreaction is unphysical and the theorem about it fails to be a mathematically general statement; (iii) the scheme that underlies the trace-free theorem does not involve averaging and therefore does not capture crucial non-local effects; (iv) their arguments are to a large extent coordinate-dependent, and (v) many of their criticisms of backreaction frameworks do not apply to the published definitions of these frameworks. It is therefore incorrect to infer that Green and Wald have proven a general result that addresses the essential physical questions of backreaction in cosmology.

## Proving Heliocentricity

Is it stupid to think that the Sun revolves around the Earth?

Well, of course anyone even slightly exposed to scientific thinking who believes that today is certifiably a fruitcake or, as Professor Brian Cox puts it “a nobber”.

But proving heliocentricity – unlike, say, the spherical nature of the Earth, is not actually all that simple at all.

The crudest evidence suggests to us that the Sun goes round the Earth once every 24 hours and it is quite easy to disprove that, but the alternative – that the Sun goes round the Earth once every year is a lot more difficult.

Painstaking collection of data about planetary movements will show they (other than Mercury and Venus) display so-called “retrograde movement” at around the time they are in opposition to the Sun (on the opposite side of the sky) – something that is much simpler to explain through heliocentricity than geocentricity – but collecting that data is not something you and I are likely to do in a hurry.

Venus and Mercury’s different behaviour does suggest they orbit the Sun and as the geocentric model came under attack in the 16th and 17th centuries that was one of the earliest concessions of geocentricity’s defenders: but even that is not definitive (remember we have no theory of gravity here and so we may posit any orbital period we like for these planets).

Even the discovery of the phases of Venus towards the end of the opening decade of the 17th Century did not completely kill the idea of geocentricity off – though it was the heaviest blow yet.

In fact the idea of geocentricity lingered on in scientific thinking for some decades. Partly that was the influence of the Catholic Church but that is not the full explanation – heliocentricity turns out to be quite hard to prove.

## Supernova in Mill Hill

Many years ago I spent a cloudy evening at the University of London’s observatory – in the heart of Mill Hill (presumably when this was built it was many miles from un-natural light and well beyond the city limits.) We ended up playing chess indoors.

Well, it seems city lights are not necessarily a barrier to discovery – as a team there seem to the ones that first spotted a supernova in M82.

## A beautiful thing no human will ever see

It must be a wonderful sight, but no human will ever see it – our Galaxy from the Large Magellanic Cloud.

Whoever wrote the Wikipedia entry did a fine job:

From a viewpoint in the LMC, the Milky Way would be a spectacular sight. The galaxy’s total apparent magnitude would be −2.0—over 14 times brighter than the LMC appears to us on Earth—and it would span about 36° across the sky, which is the width of over 70 full moons. Furthermore, because of the LMC’s high galactic latitude, an observer there would get an oblique view of the entire galaxy, free from the interference of interstellar dust which makes studying in the Milky Way’s plane difficult from Earth.[42] The Small Magellanic Cloud would be about magnitude 0.6, substantially brighter than the LMC appears to us.

## More astrophotography

The advantage of waiting a little longer to take some more shots is that the sky gets darker and you can drop the exposure time to 20 seconds. The disadvantage is that clouds appear…

But here are some more shots:

(a) Deneb, Cygnus and the Milky Way

(b) Altair and Aquila – milky way visible here

(c) Cassiopeia

(d) Cygnus again, with a better view of the Milky Way

(e) Ursa Major (Plough/Big Dipper/Great Bear)

Already looking forward to this year’s summer holiday – in a relatively Moon-free fortnight – and a chance to get the telescope out.

Last year I shocked myself with this:

– produced by just pointing the second-hand DSLR at the eyepiece. This year I want to go better but don’t want to spend a fortune either (ie., I am not buying a thousand quid camera).

I have read webcams are the way to go – can anyone offer some advice on how this all should work?

## Gamma ray bursts are not that rare

Yesterday it was reported that scientists have suggested that an anomalous peak in radioactive materials discovered in antarctic ice sediments and in ancient Japanese cedar trees could be explained by a gamma ray burst hitting the Earth in the 8th century CE.

The BBC radio report I head described gamma ray bursts as “extremely rare” and the website article – and much other coverage – repeats the idea that they are rare events.

But they are not.

There are an estimated $1.25\times10^{11}$ galaxies in the universe. It is estimated that a gamma ray burst happens at least once every million years in any galaxy – or approximately every $3\times10^8$ days. That means that today there will be approximately 1000 gamma ray bursts. Now, let’s assume that due to relativistic effects  we can at most only observe one-tenth of the universe, that still means 100 events in the observable part of the universe (how big the observable universe is in comparison to the universe is another matter however).

Of gamma ray bursts are narrowly beamed so even with this high rate of production not many get seen on Earth (probably a good thing given the energies involved), but they are far from rare.

Of course, events in our galaxy are rare (the fact that we are here at all is testament to that), but on the universal scale that is drawing a very tight boundary on the region being tested.