Archive for Dark Energy

Phlogiston, Dark Energy and Modified Levity

Posted in History, The Universe and Stuff with tags , , on May 21, 2015 by telescoper

What happens when something burns?

Had you aslked a seventeenth-century scientist that question and the chances are the answer would  have involved the word phlogiston, a name derived from the Greek  φλογιστόν, meaning “burning up”. This “fiery principle” or “element” was supposed to be present in all combustible materials and the idea was that it was released into air whenever any such stuff was ignited. The act of burning was thought to separate the phlogiston from the dephlogisticated “true” form of the material, also known as calx.

The phlogiston theory held sway until  the late 18th Century, when Antoine Lavoisier demonstrated that combustion results in an increase in weight of the material being burned. This poses a serious problem if burning also involves the loss of phlogiston unless phlogiston has negative weight. However, many serious scientists of the 18th Century, such as Georg Ernst Stahl, had already suggested that phlogiston might have negative weight or, as he put it, “levity”. Nowadays we would probably say “anti-gravity”.

Eventually, Joseph Priestley discovered what actually combines with materials during combustion:  oxygen. Instead of becoming dephlogisticated, things become oxidised by fixing oxygen from air, which is why their weight increases. It’s worth mentioning, though, the name that Priestley used for oxygen was in fact “dephlogisticated air” (because it was capable of combining more extensively with phlogiston than ordinary air). He  remained a phlogistonian longer after making the discovery that should have killed the theory.

So why am I rambling on about a scientific theory that has been defunct for more than two centuries?

Well,   there just might be a lesson from history about the state of modern cosmology. Not long ago I gave a talk in the fine city of Bath on the topic of Dark Energy and its Discontents. For the cosmologically uninitiated, the standard cosmological model involves the hypothesis that about 75% of the energy budget of the Universe is in the form of this “dark energy”.

Dark energy is needed to reconcile three basic measurements: (i) the brightness distant supernovae that seem to indicate the Universe is accelerating (which is where the anti-gravity comes in); (ii) the cosmic microwave background that suggests the Universe has flat spatial sections; and (iii) the direct estimates of the mass associated with galaxy clusters that accounts for about 25% of the mass needed to close the Universe. A universe without dark energy appears not to be able to account for these three observations simultaneously within our current understanding of gravity as obtained from Einstein’s theory of general relativity.

We don’t know much about what this dark energy is, except that in order to make our current understanding work out it has to produce an effect something like anti-gravity, vaguely reminiscent of the “negative weight” hypothesis mentioned above. In most theories, the dark energy component does this by violating the strong energy condition of general relativity. Alternatively, it might also be accounted for by modifying our theory of gravity in such a way that accounts for anti-gravity in some other way. In the light of the discussion above maybe what we need is a new theory of levity? In other words, maybe we’re taking gravity too seriously?

Anyway, I don’t mind admitting how uncomfortable this dark energy makes me feel. It makes me even more uncomfortable that such an enormous  industry has grown up around it and that its existence is accepted unquestioningly by so many modern cosmologists. Isn’t there a chance that, with the benefit of hindsight, future generations will look back on dark energy in the same way that we now see the phlogiston theory?

Or maybe the dark energy really is phlogiston. That’s got to be worth a paper!

Ned Wright’s Dark Energy Piston

Posted in The Universe and Stuff with tags , , , , on April 29, 2015 by telescoper

Since Ned Wright picked up on the fact that I borrowed his famous Dark Energy Piston for my talk I thought I’d include it here in all its animated glory to explain a little bit better why I think it was worth taking the piston.

The two important things about dark energy that enable it to reconcile apparently contradictory observations within the framework of general relativity are: (i) that its energy-density does not decrease with the expansion of the Universe (as do other forms of energy, such as radiation); and (ii) that it has negative pressure which, among other things, means that it causes the expansion of the universe to accelerate.

piston-animThe Dark Energy Piston (above) shows how these two aspects are related. Suppose the chamber of the piston is filled with “stuff” that has the attributes described above. As the piston moves out the energy density of dark energy does not decrease, but its volume does, so the total amount of energy in the chamber must increase. Since the system depicted here consists only of the piston and the chamber, this extra energy must have been supplied as work done by the piston on the contents of the chamber. For this to have happened the stuff inside must have resisted being expanded, i.e. it must be in tension. In other words it has to have negative pressure.

Compare the case of “ordinary” matter, in the form of an ideal gas. In such a case the stuff inside the piston does work pushing it out, and the energy density inside the chamber would therefore decrease.

If it seems strange to you that something that is often called “vacuum energy” has the property that its density does not decrease when it subjected to expansion, then just consider that a pretty good definition of a vacuum is something that, when you do dilute it, you don’t any less!

So how does this dark vacuum energy stuff with negative pressure cause the expansion of the Universe to accelerate?

Well, here’s the equation that governs the dynamical evolution of the Universe:

DecelerationI’ve included a cosmological constant term (Λ) but ignore this for now. Note that if the pressure p is small (e.g. how it would be for cold dark matter) and the energy density ρ is positive (which it is for all forms of energy we know of) then in the absence of Λ the acceleration is always negative, i.e. the universe decelerates. This is in accord with intuition: because gravity always pulls we expect the expansion to slow down by the mutual attraction of all the matter. However, if the pressure is negative, the combination in brackets can be negative so can imply accelerated expansion.

In fact if dark energy stuff has an equation of state of the form p=-ρc2 then the combination in brackets leads to a fluid with precisely the same effect that a cosmological constant would have, so this is the simplest kind of dark energy.

When Einstein introduced the cosmological constant in 1915/6 he did it by modifying the left hand side of his field equations, essentially modifying the law of gravitation. This discussion shows that he could instead have modified the right hand side by introducing a vacuum energy with an equation of state p=-ρc2. A more detailed discussion of this can be found here.

Anyway, which way you like to think of dark energy the fact of the matter is that we don’t know how to explain it from a fundamental point of view. The only thing I can be sure of is that whatever it is in itself, dark energy is a truly terrible name for it.

I’d go for “persistent tension”…

Dark Energy and its Discontents – the Talk

Posted in Biographical, Books, The Universe and Stuff with tags , , , on April 28, 2015 by telescoper

Yet another very busy day, so I just have time to post the slides of the talk I gave last week, on  Friday 24th April 2015, entitled Dark Energy and its Discontents, at the very posh-sounding Bath Royal Literary and Scientific Institution. Here is the poster

 

Bath_lecture

 

And here are the slides – though I didn’t get through them all on the night!…

A Happy Hubble Coincidence

Posted in Biographical, Books, The Universe and Stuff with tags , , , on April 25, 2015 by telescoper

image

Preoccupied with getting ready for my talk in Bath  I forgot t post an item pointing out that yesterday was the 25th anniversary of the launch of the Hubble Space Telescope. Can it really be so long?

Anyway, many happy returns to Hubble. I did manage to preempt the celebrations however by choosing the above picture of the Hubble Ultra Deep Field as the background fo the poster advertising the talk.

Anyway it went reasonably well. There was a full house and questions went on quite a while. Thanks to Bath Royal Literary and Scientific Institution for the invitation!

Dark Energy and its Discontents

Posted in Biographical, Books, The Universe and Stuff with tags , on April 24, 2015 by telescoper

Just time for a spot of gratuitous self-promotion. I shall be giving a public lecture tonight, Friday 24th April 2015, entitled Dark Energy and its Discontents, at the very posh-sounding Bath Royal Literary and Scientific Institution.
I am just finishing the slides for the talk, and packing some dark energy in my bag to use as a demonstration.
Here is the poster for tonight’s event, which explains all…

Bath_lecture

Perhaps I’ll see the odd reader of this blog there?

Forthcoming Attraction: Dark Energy and its Discontents

Posted in Talks and Reviews, The Universe and Stuff with tags , , on March 11, 2015 by telescoper

Busy again today, so just time for a spot of gratuitous self-promotion. I shall be giving a public lecture on Friday 24th April 2015 at the very posh-sounding Bath Royal Literary and Scientific Institution. Here is the poster, which explains all. Will I see any readers of this blog there?

Bath_lecture

Getting the Measure of Space

Posted in The Universe and Stuff with tags , , , , , , , on October 8, 2014 by telescoper

Astronomy is one of the oldest scientific disciplines. Human beings have certainly been fascinated by goings-on in the night sky since prehistoric times, so perhaps astronomy is evidence that the urge to make sense of the Universe around us, and our own relationship to it, is an essential part of what it means to be human. Part of the motivation for astronomy in more recent times is practical. The regular motions of the stars across the celestial sphere help us to orient ourselves on the Earth’s surface, and to navigate the oceans. But there are deeper reasons too. Our brains seem to be made for problem-solving. We like to ask questions and to try to answer them, even if this leads us into difficult and confusing conceptual territory. And the deepest questions of all concern the Cosmos as a whole. How big is the Universe? What is it made of? How did it begin? How will it end? How can we hope to answer these questions? Do these questions even make sense?

The last century has witnessed a revolution in our understanding of the nature of the Universe of space and time. Huge improvements in the technology of astronomical instrumentation have played a fundamental role in these advances. Light travels extremely quickly (around 300,000 km per second) but we can now see objects so far away that the light we gather from them has taken billions of years to reach our telescopes and detectors. Using such observations we can tell that the Universe was very different in the past from what it looks like in the here and now. In particular, we know that the vast agglomerations of stars known as galaxies are rushing apart from one another; the Universe is expanding. Turning the clock back on this expansion leads us to the conclusion that everything was much denser in the past than it is now, and that there existed a time, before galaxies were born, when all the matter that existed was hotter than the Sun.

This picture of the origin and evolution is what we call the Big Bang, and it is now so firmly established that its name has passed into popular usage. But how did we arrive at this description? Not by observation alone, for observations are nothing without a conceptual framework within which to interpret them, but through a complex interplay between data and theoretical conjectures that has taken us on a journey with many false starts and dead ends and which has only slowly led us to a scheme that makes conceptual sense to our own minds as well as providing a satisfactory fit to the available measurements.

A particularly relevant aspect of this process is the establishment of the scale of astronomical distances. The basic problem here is that even the nearest stars are too remote for us to reach them physically. Indeed most stars can’t even be resolved by a telescope and are thus indistinguishable from points of light. The intensity of light received falls off as the inverse-square of the distance of the source, so if we knew the luminosity of each star we could work out its distance from us by measuring how much light we detect. Unfortunately, however, stars vary considerably in luminosity from one to another. So how can we tell the difference between a dim star that’s relatively nearby and a more luminous object much further away?

Over the centuries, astronomers have developed a battery of techniques to resolve this tricky conundrum. The first step involves the fact that terrestrial telescopes share the Earth’s motion around the Sun, so we’re not actually observing stars in the sky from the same vantage point all year round. Observed from opposite extremes of the Earth’s orbit (i.e. at an interval of six months) a star appears to change position in the sky, an effect known as parallax. If the size of the Earth’s orbit is known, which it is, an accurate measurement of the change of angular position of the star can yield its distance.

The problem is that this effect is tiny, even for nearby stars, and it is immeasurably small for distant ones. Nevertheless, this method has successfully established the first “rung” on a cosmic distance ladder. Sufficiently many stellar distances have been measured this way to enable astronomers to understand and classify different types of star by their intrinsic properties. A particular type of variable star called a Cepheid variable emerged from these studies as a form of “standard candle”; such a star pulsates with a well-defined period that depends on its intrinsic brightness so by measuring the time-variation of its apparent brightness we can tell how bright it actually is, and hence its distance. Since these stars are typically very luminous they can be observed at great distances, which can be accurately calibrated using measured parallaxes of more nearby examples.

Cepheid variables are not the only distance indicators available to astronomers, but they have proved particularly important in establishing the scale of our Universe. For centuries astronomers have known that our own star, the Sun, is just one of billions arranged in an enormous disk-like structure, our Galaxy, called the Milky Way. But dotted around the sky are curious objects known as nebulae. These do not look at all like stars; they are extended, fuzzy, objects similar in shape to the Milky Way. Could they be other galaxies, seen at enormous distances, or are they much smaller objects inside our own Galaxy?

Only a century ago nobody really knew the answer to that question. Eventually, after the construction of more powerful telescopes, astronomers spotted Cepheid variables in these nebulae and established that they were far too distant to be within the Milky Way but were in fact structures like our own Galaxy. This realization revealed the Cosmos to be much larger than most astronomers had previously imagined; conceptually speaking, the Universe had expanded. Soon, measurements of the spectra of light coming from extragalactic nebulae demonstrated that the Universe was actually expanding physically too. The evidence suggested that all distant galaxies were rushing away from our own with speed proportional to their distance from us, an effect now known as Hubble’s Law, after the astronomer Edwin Hubble who played a major role in its discovery.

A convincing theoretical interpretation of this astonishing result was only found with the adoption of Einstein’s General Theory of Relativity, a radically new conception of how gravity manifests itself as an effect of the behaviour of space-time. Whereas previously space and time were regarded as separate and absolute notions, providing an unchanging and impassive stage upon which material bodies interact, after Einstein space-time became a participant in the action, both influencing, and being influenced, by matter in motion. The space that seemed to separate galaxies from one another, was now seen to bind them together.
Hubble’s Law emerges from this picture as a natural consequence an expanding Universe, considered not as a collection of galaxies moving through static space but embedded in a space which is itself evolving dynamically. Light rays get bent and distorted as they travel through, and are influenced by, the changing landscape of space-time the encounter along their journey.

Einstein’s theory provides the theoretical foundations needed to construct a coherent framework for the interpretation of observations of the most distant astronomical objects, but only at the cost of demanding a radical reformulation of some fundamental concepts. The idea of space as an entity, with its own geometry and dynamics, is so central to general relativity that one can hardly avoid asking what it is space in itself, i.e. what is its nature? Outside astronomy we tend to regard space as being the nothingness that lies in between the “things” (i.e. material bodies of one sort or another). Alternatively, when discussing a building (such as an art gallery) “a space” is usually described in terms of the boundaries enclosing it or by the way it is lit; it does not have attributes of its own other than those it derives from something else. But space is not simply an absence of things. If it has geometry and dynamics it has to be something rather than nothing, even if the nature of that something is extremely difficult to grasp.

Recent observations, for example, suggest that even a pure vacuum of “empty space” possesses “dark energy” energy of its own. This inference hinges on the type Ia supernova, a type of stellar explosion so luminous it can (briefly) outshine an entire galaxy before gradually fading away. These cataclysmic events can be used as distance indicators because their peak brightness correlates with the rate at which they fade. Type Ia supernovae can be detected at far greater distances than Cepheids, at such huge distances in fact that the Universe might be only about half its current size when light set out from them. The problem is that the more distant supernovae look fainter, and consequently at greater distances, than expected if the expansion of the Universe were gradually slowing down, as it should if there were no dark energy.

At present there is no theory that can fully account for the existence of vacuum energy, but it is possible that it might eventually be explained by the behaviour of the quantum fields that arise in the theory of elementary particles. This could lead to a unified description of the inner space of subatomic matter and the outer space of general relativity, which has been the goal of many physicists for a considerable time. That would be a spectacular achievement but, as with everything else in science, it will only work out if we have the correct conceptual framework.

 

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