Tag Archives: dark energy

Nobel Prize for Higgs

On 8 October 2013, the Nobel prize for physics was awarded to Francois Englert and Peter Higgs. In one sense this was a long time coming: the theoretical work that won the prize took place in 1964 (Englert, and his late colleague Robert Brout, working independently of Higgs, published first; a few weeks later Higgs published a paper that explicitly predicted the existence of a scalar boson; another group of physicists – Gerald Guralnik, Carl Hagen and Tom Kibble – published related work later in the same year). In another sense the prize was awarded remarkably quickly: experimental proof of the existence of a fundamental boson was announced on 4 July 2012, and it wasn’t until 14 March 2013 that it was confirmed to be a scalar (spin-0) boson. (If you want to learn more about the Higgs mechanism, you can find a variety of explanations here.)

To my mind, the discovery of the Higgs is one the crowning achievements of human civilisation: it is the culmination of a process that began 2500 years ago with the Greeks. Physicists now have a standard model of fundamental particles: there exists a small number of spin-1/2 point particles (6 quarks; the electron, muon and tau each with their associated neutrino) which interact via the exchange of spin-1 particles that mediate the electroweak and strong (these exchange particles being the photon; W+, W and Z0; 8 gluons). In the ‘pure’ theories underpinning this model the fundamental particles are massless; they acquire mass – and thus in a certain sense their very existence – by interacting with a spin-0 field that pervades the entire universe. This spin-0 field has an associated particle; the Higgs boson. And that’s it. End of story. Except…

We are really just at the beginning of the story. The theories underpinning the standard model are in conflict with the other central pillar of physics: general relativity. The standard model is based on quantum physics; general relativity is a classical theory. Physicists need to develop a quantum theory of gravity. Furthermore, we now know that the standard model applies to only 5% of the universe: 95% of the mass-energy content of the universe resides in the so-called ‘dark’ sector. We desperately need to understand the nature of dark matter and dark energy.

Now that the Large Hadron Collider has discovered the Higgs its next job, when it becomes operational again after its current upgrade, is to shed light on the dark sector.


Planck results are here!

This morning an ESA press conference presented results from an analysis of the first 15 months of data from the Planck mission. The results are exquisite, and it’s clear that Planck will be as important for cosmology as its predecessors COBE and WMAP. Cosmologists will be poring over the data for years to come.

I’ll give more detail in future posts, but for the moment here are just two items.

First, the most detailed picture yet of the early universe:

Planck's stunning new map of the universe

Planck’s stunning new map of the universe (Credit: ESA)

Second, some of the stand-out points from this morning’s presentation:

  • The universe is slightly older than we previously thought (about 80 million years older in fact): it’s 13.82 billion years old.
  • Planck measures the Hubble constant to be 67 km s-1 Mpc-1. This is slightly smaller than most other recent estimates. Curious!
  • The energy inventory of the universe isn’t quite what we thought it was: there’s slightly more dark matter than previously thought and slightly less dark energy. The universe is currently made up of 4.9% normal matter, 26.8% dark matter and 68.3% dark energy.
  • On small scales, the standard cosmological model (which includes inflation) agrees supremely well with the observed cosmic microwave background. The standard cosmological model is in good shape.
  • There are hints, based on observations of the largest angular scales, of physics beyond our current theories. In particular: (i) the sky in the southern hemisphere is ever so slightly warmer than the sky in the northern hemisphere; (ii) large-scale temperature fluctuations are weaker than expected; and (iii) there’s a cold spot in the universe, in the constellation Eridanus, that’s much larger than our models would predict. Gaining an understanding of these anomalies is going to lead to some really interesting ideas over the next few years.

SZ effects

In Measuring the Universe I talked about the Sunyaev-Zel’dovich effect (or the SZ effect, for short). It’s named after Rashid Sunyaev and Yakov Zel’dovich, who studied the concept in the late 1960s and early 1970s.

The SZ effect is a distortion in the observed cosmic microwave background radiation caused by high-energy electrons scattering of low-energy CMB photons. The collisions give the photons an energy boost – it’s the familiar inverse Compton scattering effect – and this in turn generates a slightly hotter patch in the microwave background. (‘Slightly’ is the operative word here: a microwave photon passing through a cloud of hot electrons on its journey towards Earth will appear hotter by just a few millionths of a degree.)

The high-energy electrons that can cause the SZ effect are to be found in the extremely hot gas clouds that are found at the centre of galaxy clusters. And, because the SZ effect is caused by scattering, its size doesn’t depend on redshift. In other words, the SZ effect in a high-redshift cluster can be detected just as easily (or, more truthfully, with just as much difficulty!) as in a cluster at low redshift. The SZ effect provides what is in essence a standard ruler – see Measuring the Universe for details – and so it can be used as a distance indicator over quite large reaches of the cosmos.

But there’s another type of SZ effect – the so-called kinematic SZ effect. I didn’t bother discussing it in the book because it is about 20 times fainter than the main (or thermal) SZ effect. Since the thermal SZ effect is hard enough to measure I didn’t think that anyone would be measuring the kinematic SZ effect anytime soon. Well, I was wrong. Cosmologists have now measured it.

The kinematic SZ effect arises because of the motion of galaxy clusters. Imagine a CMB photon passing through a cluster that’s moving away from us: when we observe the photon it will be slightly cooler (redder) than it otherwise would be due to the kinematic SZ effect. And if the photon moves through a cluster that’s approaching us then it will be slightly hotter (bluer). Sunyaev and Zel’dovich considered this from a theoretical point of view four decades ago, in 1972; but it’s taken until 2012 for researchers to measure it, such is the difficulty of teasing out the signal.

Kinematical SZ effect

If a CMB photon passes through a galaxy cluster that’s moving away from Earth it becomes slightly redder and cooler (left part of the diagram). If it passes through a galaxy cluster that’s moving towards Earth then it becomes bluer and hotter (right part of the diagram). These wavelength shifts are extremely tiny, so this effect has only just been observed.
(Credit: Sudeep Das, University of California-Berkeley)

A paper by Nick Hand (and about 60 other scientists, who were part of the Atacama Cosmology Telescope and the Baryon Oscillation Spectroscopic Survey projects), called Detection of Galaxy Cluster Motions with the Kinematic Sunyaev-Zel’dovich Effect, has identified the local velocity of galaxy clusters at a distance of up to several billion light years. Because the kinematic SZ effect is independent of redshift (in the same way that the thermal SZ effect is independent of redshift) cosmologists now have a tool for measuring velocities as well as distances way out into the cosmos.

The SZ effects can probe how clusters form and move around – something that depends critically on dark matter and dark energy. The SZ effects thus have the potential to deepen our understanding of the most mysterious elements of our Universe.

Euclid gets the go-ahead

One of the missions I talk about in New Eyes on the Universe is the Euclid telescope. When I wrote the book, Euclid was still a mere possibility. Today, Euclid took one step closer to becoming  a reality.

The Science Programme Committee of ESA today formally adopted the mission – meaning that the money is in place to proceed. (In the current economic climate that is a significant step. To build, launch and operate Euclid is going to cost ESA in the region of half a billion pounds; you can add an extra 150 million on top of that for the cameras and spectrometers that Euclid will use.)

The Euclid telescope in space

Euclid at Lagrange Point 2 (Credit: ESA)

And what will we get for all this money? Hopefully, a much better understanding of what dark energy is. Euclid will have a 1.2m mirror and three scientific instruments for observing in the optical and infrared. Its observations will cover half of the entire sky and it will map structures from ten billion years ago up to the present day. The idea is that it will measure the distances to objects using three different methods: primarily through baryon acoustic oscillations but also through observations of type Ia supernovae and weak gravitational lensing. By combining this distance data with redshift data, cosmologists can determine the ‘size’ of the universe at different times in its history. In other words, they can determine the expansion history of the universe. And it’s the careful study of this expansion history that will enable cosmologists to learn more about the mysterious quantity – dark energy – that’s causing the expansion to accelerate.

Euclid should launch in 2019. So in ten years or so we might be better able to answer one of the most puzzling questions in science: what’s causing the universe to blow itself apart?



A new standard candle?

Standard candles have played a hugely important role in establishing the cosmological distance ladder. It’s easy to see why: the more distant something is the dimmer it appears, according to the inverse-square law. So if we know how bright something really is then, by measuring how bright it appears to be, we can determine its distance.

A standard candle appears dimmer the more distant it is

A standard candle appears dimmer the more distant it is
Credit: Karen Kwitter

Cepheid variables and Type Ia supernovae are perhaps the most well-known standard candles, and the study of these objects have transformed our understanding of the universe. But they (and the several other standard candles used in astronomy) are not without problems. One of the main difficulties is that we can’t see them over very large distances. Even Type Ia supernovae cannot be used to make reliable distance measurements beyond a redshift of about 1.7. So one of the most interesting astronomical results of 2011, at least in my opinion, was the surprising discovery of a standard candle that can work over truly cosmological distance scales: active galactic nuclei (AGNs).

Artist's impression of an accretion disc and torus around a black hole

An artist's impression of an accretion disc and torus around an AGN
Credit: NASA/CXC/M.Weiss

An AGN is one of the brightest objects in the universe and so can be seen over extreme distances. The power source for an AGN’s extreme luminosity is the supermassive black hole that lies at its centre. An accretion disc – a collection of matter that forms as matter spirals into a dense object – surrounds an AGN’s supermassive black hole. (See chapter 4 of New Eyes on the Universe for an explanation of accretion discs.) Further away from the black hole, at least with type-1 AGNs, lies a dense area of dust and gas known as the broad-line region. The region gets its name because the black hole’s gravitational influence whips the dust and gas around at high speed, and the Doppler effect causes emission lines to be broadened.

And how does this rather chaotic set-up generate a standard candle? Well, the broad-line region emits light because its gas has been ionised. The ionisation occurs because high-energy photons are emitted by the accretion disc and subsequently hit the region. The key point here is an accretion disc is a variable object: sometimes it ‘flares’. This makes it possible to compare the time at which the accretion disc emits light and the broad-line region re-emits light, and that time delay gives the radius of the broad-line region. What four astronomers – Darach Watson, Kelly Denney, Marianne Vestergaard and Tamara Davis – have found is that there’s a relationship between the size of the radius and the central luminosity of the AGN. They checked the relationship on a sample of 38 AGNs at a known distance and it seems that, although there is scatter in the data, the technique will work as a distance indicator. (You can read their paper at arxiv.)

The AGN standard candle is not as accurate as the Cepheid or supernova candles. But since AGNs can be seen over tremendous distances, and since they can be studied over long periods of time, it seems certain that the technique will become of increasing importance. In particular, a standard candle that lets astronomers measure distances directly up to a redshift of about 4 will provide a valuable tool for probing the nature of dark energy.