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.


The gravitational perpetual motion machine?

English: Gravitational potential is a scalar p...
English: Gravitational potential is a scalar potential energy per unit mass at each point in space associated with the force fields. : \phi = -( \frac{GM}{r}) . In this case, M = 100 000 kg, G = -6.67E-11 (Photo credit: Wikipedia)

Perhaps I should have thought about this years ago – when I was surrounded by physicists and cosmologists and the like, and so could have got an answer: but I didn’t – until I started reading Lee Smolin‘s Time Reborn: From the Crisis in Physics to the Future of the Universe.

Here’s the issue: as Smolin states, nothing stops gravity. You cannot muffle it or block it.

Then let us think of an object with mass – it sends out “gravitons” (we assume – these have never been detected of course) and these exert a force on every object they meet. If we go one step further and suggest that the universe is infinite, do we not end up with our massive body being the source of an infinite amount of force and hence an infinite amount of energy?

Getting into murky waters here – but as I understand it, physics gets round this with something of an accounting trick: bodies with gravitational potential energy are deemed to have negative energy and so all that happens is that our massive body converts this negative energy into a positive energy and the total amount of energy in the universe is unchanged.

Is that really it?

An unchanging quantum universe

This another one of those bizarre thoughts that cosmology throws up which manages to be both simple and profound.

Imagine the wave function for the whole universe.

By its nature the universe cannot change its quantum state: it’s the ultimate closed system. Of course there is a  probabilistic distribution of energy inside the system but the total energy of the system does not change and therefore its quantum state cannot change either.

So, in quantum terms the universe is unchanging over time.

A probably entirely naïve question about the principle of relativity

English: M&M-experiment
English: M&M-experiment (Photo credit: Wikipedia)

Surely I can quite easily design an experiment that shows the relativity principle is false.

If turn around on the spot the principle, as I understand it, asserts that I cannot build an experiment that proves it was me that moved as opposed to everything else that moved while I stayed still.

But the rest of the universe is very massive – possibly of infinite mass – and so to move it through 2\pi radians takes a hell of a lot more energy than moving me.

Copernicus was wrong (maybe)

The Hubble Deep Field South looks very similar...
The Hubble Deep Field South looks very similar to the original HDF, demonstrating the cosmological principle. (Photo credit: Wikipedia)

No, I haven’t taken leave of my senses and decided the Sun moves around the Earth (but here is a pop quiz for all of you laughing at that idea – can you think of a simple experiment that would prove to a 10-year-old that the Earth moves around the Sun?).

In fact the issue here – the so-called Copernican Principle, or in its grander form the Cosmological Principle – was almost certainly not Copernicus’s view at all. But his paper – De Revolutionibus – opened the door to it, and indeed to the positivist idea of science in general.

The Copernican Principle states that there is nothing special about the position of Earth, the grander Cosmological Principle states that, at a sufficiently large scale, the universe looks the same in all directions – including (and this is important for what is coming) from where we are. In other words not just Earth is nothing special, but nowhere is anywhere special.

But what is that is all wrong? A fascinating article in this week’s New Scientist looks at just this.

That there are limits to the Cosmological Principle is obvious – the world is not smooth, even at some very large scales (look at the night sky – most of it is dark).

The standard scientific response to this is to state that at a sufficiently large scale – around 400 million light years – the matter density between galaxies and the inter galactic void evens out. But that assumption is based on our observations of our locality: yet what if, actually, we were in an atypical part of the universe? The atypicality could even be meta-typical (in other words we could have a super-Cosmological Principle but accepted that the universe was lumpy.)

This matters because our cosmological models are based on the assumption that the Cosmological Principle is correct and that therefore we are typical observers of the universe: hence the phenomena we see are ones that would be seen by any observer and are therefore artefacts of the universe’s physical nature and not our observational position.

So, for instance, we have data that appear to show that the expansion of the Universe is speeding up. We do not know why this is, so we call it “Dark Energy“. But what if the apparent speed up was because, actually, the universe was not isotropic (did not look the same in all directions) and the additional mass in one direction was impacting on the perceived rate of expansion of the universe?

The beauty of this question is that asking it does not mean challenging Einstein’s General Relativity – it’s not an exercise in metaphysical speculation but an argument firmly within the positivist realm bequeathed to us by Copernicus in the first place.

And finally…It is actually quite tough to devise a simple experiment to show that the Earth revolves round the Sun – but the orbits of Venus and Mercury are probably the best examples: these planets are never in opposition to the Sun. Though binoculars to observe the planets’ phases are probably needed to fully escape any Ptolemaic theories’ grasp.

Finally, finally… this book – Can You Speak Venusian? A Trip Through the Mysteries of the Cosmos – was very funny when I read it about 35 years ago, whether it has stood the test of time I am not sure.

Why we’ll never meet aliens

First page from the manuscript explaining the ...
First page from the manuscript explaining the general theory of relativity (Photo credit: Wikipedia)

Well, the answer is pretty plain: Einstein‘s theory of general relativity – which even in the last month has added to it’s already impressive list of predictive successes – tells us that to travel at the speed of light a massive body  would require an infinite amount of propulsive energy. In other words, things are too far away and travel too slow for us to ever hope to meet aliens.

But what if – and it’s a very big if – we could communicate with them, instantaneously? GR tells us massive bodies cannot travel fast, or rather along a null time line – which is what really matters if you want to be alive when you arrive at your destination – but information has no mass as such.

Intriguingly, an article in the current edition of the New Scientist looks at ways in which quantum entanglement could be used to pass information – instantaneously – across any distance at all. Quantum entanglement is one of the stranger things we can see and measure today – Einstein dismissed it as “spooky interaction at a distance” – and essentially means that we can take two similar paired particles and by measuring the state of one can instantaneously see the other part of the pair fall into a particular state (e.g., if the paired particles are electrons and we measure one’s quantum spin, the other instantly is seen to have the other spin – no matter how far away it is at the time).

Entanglement does not allow us to transmit information though, because of what the cosmologist Antony Valentini calls, in an analogy with thermodynamic “heat death”, the “quantum death” of the universe – in essence, he says that in the instants following the Big Bang physical particles dropped into a state in which – say – all electron spins were completely evenly distributed, meaning that we cannot find electrons with which to send information – just random noise.

But – he also suggests – inflation – the super-rapid expansion of the very early universe may also have left us with a very small proportion of particles that escaped “quantum death” – just as inflation meant that the universe is not completely smooth because it pushed things apart at such a rate that random quantum fluctuations were left as a permanent imprint.

If we could find such particles we could use them to send messages across the universe at infinite speed.

Perhaps we are already surrounded by such “messages”: those who theorise about intelligent life elsewhere in the universe are puzzled that we have not yet detected any signs of it, despite now knowing that planets are extremely common. That might suggest either intelligent life is very rare, or very short-lived or that – by looking at the electromagnetic spectrum – we are simply barking up the wrong tree.

Before we get too excited I have to add a few caveats:

  • While Valentini is a serious and credible scientist and has published papers which show, he says, the predictive power of his theory (NB he’s not the one speculating about alien communication – that’s just me) – such as the observed characteristics of the cosmic microwave background (an “echo” of the big bang) – his views are far from the scientific consensus.
  • To test the theories we would have to either be incredibly lucky or detect the decay products of a particle – the gravitino – we have little evidence for beyond a pleasing theoretical symmetry between what we know about “standard” particle physics and theories of quantum gravity.
  • Even if we did detect and capture such particles they alone would not allow us to escape the confines of general relativity – as they are massive and so while they could allow two parties to theoretically communicate instantly, the parties themselves would still be confined by GR’s spacetime – communicating with aliens would require us and them in someway to use such particles that were already out there, and perhaps have been whizzing about since the big bang itself.

But we can dream!

Update; You may want to read Andy Lutomirski’s comment which, I think it’s fair to say, is a one paragraph statement of the consensus physics. I am not qualified to say he’s wrong and I’m not trying to – merely looking at an interesting theory. And I have tracked down Anthony Valentini’s 2001 paper on this too.

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Review of “The Black Cloud”

Cover of "The Black Cloud"

My interest in astronomy and astrophysics comes from childhood and when I was much, much younger I had an (unscientific) fondness for the “steady state” theory of cosmology, which, in the early 1970s was not as thoroughly discredited as it is today (though, of course, many newer cosmologies borrow from it or show similarities to it – for instance Roger Penrose‘s proposal in Cycles of Time).

Part of the attraction of the steady state cosmology was the figure of Fred Hoyle, or at least how I imagined him: blunt speaking, no nonsense, scientific genius. But until I read The Black Cloud I had not read any of his scientific or literary works.

The book is fascinating as a period piece and not a bad read as a piece of science fiction either – though the overall tone and dialogue reminds me of “Journey into Space” – a 1950s BBC science fiction radio serial recently re-broadcast.

But here is a novel with differential equations, computer program listings (presumably in machine code of some sort as it certainly is not a high level language) and a description of the (then) pioneering technology of pulse code modulation.Not all the science is good though, but Hoyle cannot be blamed for that: although it does not use the term, the view of artificial intelligence here is the conventional one of the time, but also one that fifty years of rapidly advancing computing power has failed, thus far at least, to sustain.

Mixed, with that, though is a fair dose of of Little Englandism, enormous doses of sexism and a quite frightening view into how Hoyle thinks society should be organised – namely with politicians, the people we chose, removed and the dictatorship of the scientists instituted. Stalin ruled in the name of science too.

Hoyle’s preface implies it would be a mistake to ascribe the views of Chris Kingsley, the chief advocate of crushing politics, to himself, but the character sounds far too much like him – the man who once exploded with anger when a snotty PhD student called Stephen Hawking pointed out a flaw in his calculations because he thought it would weaken his attempted blackmail of politicians – for the denial to be credible.

It’s a great book and an easy read, so I do recommend it.