Posts Tagged ‘Gravitational Waves’

My latest book, Astrophotography, is now available. You can order a copy by following this link. Astrophotography is a book of exquisite images of space, including some of the latest images such as New Horizons’ images of Pluto, Rosetta’s images of Comet 67P, and Hubble Space Telescope images of the most distant galaxies ever seen. Each stunning image, reproduced to the highest quality, is accompanied by text that I have written to explain the object, and any background science relating to the object.


Astrophotography is now available. You can order your copy by following this link.

One unique aspect of Astrophotography is that it emphasises the multi-wavelength approach taken to understanding astronomical objects. For millennia we could only study the Universe in visible-light (the light to which are eyes are sensitive), but for the last few decades we have used every part of the electromagnetic spectrum from radio waves to gamma rays to better understand the Universe. This multi-wavelength approach has also enabled us to discover previously unknown aspects of the Universe such as the Cosmic Microwave Background, the true appearance of Venus’ surface which lies hidden below its thick atmosphere, and huge quantities of gas between galaxies (the intracluster medium) which emit no visible-light but prodigious amounts of X-rays.

Astrophotography is split into 5 sections, namely

  1. Exploring the Solar System
  2. Exploring the Milky Way
  3. Exploring the Local Group
  4. Beyond the Local Group
  5. At the Edge of the Universe

Below are examples of some of the beautiful images found in Astrophotography, along with examples of the accompanying text. At the beginning of each page’s text I caption which telescope or space probe has taken the main image, and at which wavelength (or wavelengths).

Exploring the Solar System

Two examples from the first section of Astrophotography, the section on the Solar System, are stunning images of Mercury and of Mars. The images of Mercury were taken by NASA’s MESSENGER spacecraft. There are several pages of images of Mars, the page shown below shows an image of the Martian surface taken by the Mars Curiosity Rover, and an image of Victoria Crater taken by the Mars Reconnaissance Orbiter.


Images of Mercury taken by NASA’s MESSENGER spacecraft. The four main images are spectral scans, and show information on the chemical composition of Mercury’s surface.

The section on the Solar System also includes images of Pluto taken by New Horizons, images of Saturn and Titan taken by the Cassini space probe, images of Comet 67P taken by Rosetta, and images of Jupiter and her moons taken by the Galileo space craft.


The surface of Mars as imaged by NASA’s Mars Curiosity Rover and, at right, Victoria Crater, as imaged by NASA’s Mars Reconnaissance Orbiter.

Exploring the Milky Way

The second section of Astrophotography includes images of the Orion Nebula (Messier 42), the reflection nebula Messier 78, the Horsehead Nebula, the Pillars of Creation (part of the Eagle Nebula), and the Crab Nebula, the remnant of a supernova which exploded in 1054.

The example I show below is of the reflection nebula Messier 78, and is a visible light image taken by the Max Planck Gerzellschaft Telescope, a 2.2 metre telescope located at the European Southern Observatory’s facility in La Silla, Chile. The text describes the history of observing Messier 78, and explains what produces a reflection nebula.


The reflection nebula Messier 78 imaged in visible light by the Max Planck Gesellschaft Telescope. The text explains what reflection nebulae are, and the history of observing this particular object.

Exploring the Local Group

The third section of Astrophotography looks at the Local Group, our part of the Universe. The Local Group includes our Milky Way galaxy, the Large and Small Magellanic Clouds, and the Andromeda galaxy. Some of the images shown in this section include the Tarantula Nebula in the Large Magellanic Cloud, NGC 602 (in the Small Magellanic Cloud), the Andromeda galaxy, Supernova 1987A and the Seahorse Nebula.

The example I show here is the Seahorse Nebula, a dark cloud of gas and dust located in Large Magellanic Cloud. This Hubble Space Telescope image was taken in 2008, and the nebula is in the bottom right of the image.


The Seahorse nebula is a dark cloud of gas and dust found in the Large Magellanic Cloud, an irregular galaxy visible to the naked eye and in orbit about our Milky Way galaxy. The seahorse nebula is in the bottom right of the image.

Beyond the Local Group

The fourth section of Astrophotography looks at the rich variety of galaxies found beyond our own neighbourhood. Examples are galaxies like Messier 82, which is undergoing a huge burst of star formation in its centre, Centaurus A, which shows huge lobes of radio radiation stretching far beyond the stars we see in visible light, colliding galaxies such as The Antennae galaxies, and evidence for dark matter such as the Bullet cluster.

The example I have shown here is the spread for Messier 81, a beautiful spiral galaxy found in Ursa Major. It is one of the best known galaxies in the sky, and is visible to northern hemisphere observers throughout the  year. The main image illustrates the multi-wavelength approach astronomers take to studying many objects. The image combines visible light, infrared light and ultraviolet light to teach us far more about the galaxy than we would learn if we only looked in visible light.


Messier 81 is a beautiful spiral galaxy found in Ursa Major. Hence it is visible throughout the year to northern hemisphere observers. The main image shown here is a combination of of a visible light image (taken by the Hubble Space Telescope), an infrared image taken by the Spitzer Space Telescope, and an ultraviolet image taken by Galaxy Evolution Explorer (GALEX).

At the edge of the Universe

In the final section of Astrophotography, I show examples of some of the most distant objects known. Images include the Hubble Deep Field, the Cosmic Microwave Background, the most distant galaxy seen (GN-z11, lying about 13.4 billion light years away), gravitational lenses and the recent discovery of gravitational waves made by LIGO.

The example I show here is the spread about the gravitational lens SDP81, a galaxy lying about 12 billion light years away which is being lensed (and brightened) by an intervening cluster of galaxies which lie about 4 billion light years away. The top image was taken at millimetre wavelengths by the Atacama Large Millimetre Array (ALMA), the bottom image in visible light by the Hubble Space Telescope.


Gravitational lenses enable us to see distant galaxies which would otherwise be too faint to see, but they also provide us with a way of tracing the distribution of dark matter in clusters.

I hope these few examples from Astrophotography have whetted your appetite to find out more. I really enjoyed putting the book together, and am very pleased with the quality of the images and their aesthetic beauty.


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On Wednesday of last week (15 June 2016), it was announced by the LIGO-Virgo collaboration that they had made their second detection of gravitational waves. This follows the announcement made by the same team in February of the first ever detection of the waves predicted by Einstein 100 years ago (see my blogposts here and here about that).

The fact that LIGO has now detected two gravitational wave events in the space of a few months suggests that there will be many more; and really does highlight how we are opening up a whole new window on the Universe, as I have said before. To me, this is akin to the development of radio astronomy in the 1950s or X-ray astronomy in the 1960s, when new sources were being detected several times a year. Or, one could say, to the development of the telescope in the early 1600s.

I think that we can not only expect to see more and more detections coming from the LIGO-Virgo team, but also an increase in sensitivity of the detections as time goes on. Even with ground-based detectors I expect the sensitivity to increase, but once we start doing this from space (as ESA plans to do with eLISA), the sensitivity will increase hugely.


The LIGO collaboration has detected a second emission of gravitational waves. This detection, announced last week (15 June 2016), was made on 25 December 2015.

As with the first detection, this second detection seems to be of two black holes merging. However, unlike the first event, which lasted about a tenth of a second, this event was about 1-second long. Also, whereas it is calculated that the two black holes in the September 2015 event (announced in February) had masses of 29 and 36 times the mass of the Sun, the black holes in this event had masses of 11 and 8 times the mass of the Sun. It is the lower mass of the two black holes in this second event which leads to the merger taking longer, as their orbits about each other would have been slower.

We really are living at a very exciting time, to be witnessing this whole new window on the Universe opening up.

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A new window on the Universe

Last Thursday (11 February 2016) the very exciting news broke that scientists had directly detected gravitational waves for the very first time. I briefly blogged about it here, but now that I am back from giving talks in South America I have a little more time (and a much better internet connection) to write a more complete blogpost about it.

This direct detection, hopefully the first of many, was made by two international teams (including colleagues of mine at Cardiff University) using the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is in the United States. The link to the actual paper, which appears in Physical Review Letters, is here. As you can see from the abstract of the paper (see below), the signal was detected by both LIGO detectors simultaneously, which is a very important point as it strongly suggests that the detection is real.


The abstract of the paper by Abbott et al. announcing the first ever detection of gravitational waves (from Physical Review Letters)

If you read the last sentence of the abstract, you will notice that it states that not only is this the first ever direct detection of gravitational waves, but is also the first ever observation of two black holes merging. This illustrates nicely that being able to detect gravitational waves opens up a whole new way of learning about the Universe; so this detection marks a significant step forward in our capabilities to “observe” our Universe. In my mind, it is as significant as William Herschel’s accidental discovery of infrared light in 1800, which was the first ever detection of radiation outside of the visible part of the spectrum.

What are gravitational waves?

Gravitational waves were predicted by Albert Einstein in 1916, as a natural consequence of his new theory of gravity – general relativity. Unlike Newton’s theory of gravity, which argued that gravity acts instantaneously, Einstein’s general theory of relativity predicts that gravity is due to a bending of spacetime. The bending (warping) is produced by masses (e.g. the Sun, black holes, galaxies, clusters of galaxies); and changes in any gravitational field travel through space at the speed of light from the place of change as ripples , distorting spacetime as they spread outwards.

Although predicted one hundred years ago by Einstein’s theory, up until now there has only been indirect evidence for gravitational waves. The best example of indirect evidence is from observations of pulsars, spinning neutron stars. Back in 1974 it was noticed by Russell Hulse and Joseph Taylor that the period of a particular pulsar, PSR B1913+16 (which happens to be one of a pair of orbiting neutron stars) was slowing down. It was argued that this was due to the neutron stars spiralling towards each other and losing energy, with this energy being taken away in the form of gravitational waves. The calculations matched, the loss of energy being measured agreed with the predicted energy which should be carried away as gravitational waves. Hulse and Taylor received the 1993 Nobel Prize in Physics for this work.

Since then, there have been other similar observations, all agreeing with the predictions of general relativity; but until now there had been no direct observations of the elusive gravitational waves.

‘Seeing’ back to the beginning of time

The cosmic microwave background, the subject of my recent book, comes about from when the Universe became transparent.

My book "The Cosmic Microwave Background - how it changed our understanding of the Universe" is published by Springer

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

But, when I say ‘transparent’, I mean transparent to electromagnetic (EM) radiation. Gravitational waves are not EM radiation, so the opacity of the Universe before about 400,000 years into its existence does not apply to gravitational waves. These should be detectable back to the very tiniest fraction of the first second, a time when gravity and the other three forces separated. The ‘surface of last scattering’ (from where the CMB comes), effectively acts as a wall to EM radiation,  but it is transparent to gravitational waves.

We have a very long way to go before we are able to detect gravitational waves directly from anything but nearby objects. But, the potential is there at some point in the future to be able to use gravitational waves to probe back to the beginning of time. That is a truly exciting possibility!

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A major science story broke on Thursday (11th of February 2016) – the first ever detection of gravitational waves. I am currently in South America giving astronomy talks on a cruise, and therefore my internet access is very slow and very very expensive.

So, I will do a more detailed blogpost about this later next week, hopefully by Tuesday (17th). In the meantime, you can find out more about this very exciting announcement and what it means for astronomy by going to your favourite news website. From the little that I have been able to see, the coverage and explanations on the BBC’s science pages is hard to beat. 

More next week!!

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A few hours ago the European Space Agency (ESA) launched a satellite which will hopefully lead to our being able to detect ripples in space. The LISA Pathfinder satellite took off from French Guiana just after 4am GMT, and its purpose is to test the feasibility of a far more ambitious experiment called LISA which will be launched in the 2030s.  

An artist’s impression of the LISA Pathfinder satellite


The satellite will monitor the separation between two gold-platinum blocks

 There are ground-based experiments to detect gravitational waves (the ripples in space that I referred to above), but doing such experiments from space should provide more sensitivity. These waves are a prediction of Einstein’s theory of gravity, the general theory of relativity. We are yet to detect any gravitational waves, but as I explain in my book on the cosmic microwave background, detecting them will provide us with another way of measuring properties of the Universe. In some ways, rather than seeing the Universe, we would be able to feel it. 

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To most of us, inflation is a nasty thing which sees the money in our pocket being worth less as prices go up. It’s a bad thing! But, in cosmology, a theory called cosmic inflation explains very neatly several key properties of the Universe. The theory of cosmic inflation was first suggested by Alan Guth in 1980, and yesterday (Monday the 17th of March 2014) a team led by John Kovak of Harvard University announced the first direct evidence that cosmic inflation did actually happen. There is also a Cardiff University involvement in this project.

The story on the confirmation of cosmic inflation as it appeared on the BBC science website.

The story on the confirmation of cosmic inflation as it appeared on the BBC science website.

What is cosmic inflation?

In 1980, particle physicist Alan Guth was pondering some of the observed properties of the Universe, and he came up with the idea of cosmic inflation. The observed properties he was hoping to explain with his theory were

  • the “Horizon problem”
  • the “Flatness problem”
  • the “Magnetic-monopole problem”

The Horizon problem

When the cosmic microwave background radiation (CMBR) (the prediction of which I blogged about here) was discovered in 1964 it was recognised that it was most probably the “echo” of the Big Bang. By 1967 Bruce Partridge and David Wilkinson of Princeton University showed that the CMBR was the same from all parts of the sky down to a level of 0.1% of its 3 Kelvin temperature.

It was realised soon after this that this presented a problem, the so called “horizon problem”. It is actually perplexing that different parts of the sky should have the same CMBR temperature because when we look in different parts of the sky we are looking at parts of space which have not had the time to be in contact with each other in any way; they are simply too far apart. Therefore, a patch of sky in one direction with a particular CMBR temperature should have no knowledge of the CMBR temperature of a patch of sky in a different direction.

This is a little bit like switching on a heater in the centre of a large room. Everyone knows that it will take time for the whole room to come to the same temperature, and if the room were really really big you would not expect the corners which are far away from the heater to have the same temperature as the centre of the room next to the heater after just a few minutes. The heat just hasn’t had enough time to spread throughout the room. So, if you found that the whole room was at the same temperature, even though the heat hadn’t had enough time to spread throughout the room, it would be a bit of a puzzle. That is, in essence, the “horizon problem”.

The flatness problem

Einstein showed in his theory of gravity, the General Theory of Relativity, that gravity causes space to bend. A Universe with lots of matter in it will have a different geometry (shape) to a Universe with less matter in it. The so-called “critical density” of the Universe would be a density that would give it a flat geometry. It was realised since the 1960s that the density of the Universe seemed to be very close to the critical density. Why should this be, when it could have any value. It could be much much more or much much less? If you do the mathematics, for the density to be within about a factor of two of the critical density today means it had to have been incredibly close to the critical density in the earliest moments of the Universe. Close to about one part in 10^{60}!! This is the “flatness problem”.

The magnetic monopole problem

In electricity, we are all familiar with positive and negative charges. James Clerk Maxwell showed in the mid 1800s that electricity and magnetism are part of the same force, electromagnetism. And yet, you never find a magnetic monopole, you always find magnetic poles come in pairs, they always have both a north and south pole. Theoretically there is no reason why one shouldn’t find just e.g. a north pole on its own, without a south pole. This is the “magnetic monopole problem”.

What is cosmic inflation?

Alan Guth’s idea of cosmic inflation suggested that when the Universe was incredibly young, some 10^{-36} seconds old, it went through a brief period of very rapid expansion. This period ended when the Universe was about 10^{-33} \text{ or } 10^{-32} seconds old, but in this incredibly brief period Guth argued that the Universe grew from being much smaller than a proton to something about the size of a marble. After this brief period of very rapid expansion (inflation), the expansion of the Universe settled down to the more sedate rate of expansion that we see today.

How does cosmic inflation solve these three problems?

The horizon problem is solved by inflation because the very rapid expansion which inflation proposes would allow parts of the Universe which are now too far apart to have ever communicated with each other to have been close enough together before inflation. So, going back to my analogy with the room being heated, it is as if the room started off really small, so small that all parts of it could come to the same temperature, then it suddenly expanded so that the room we are now looking at is much much bigger.

The flatness problem is solved by cosmic inflation by drawing the analogy between the geometry of the Universe and a curved surface. If a curved surface is large enough, then on a local scale it is always going to look flat. An easy analogy to understand this is the surface of our Earth. We all know it is spherical, but on a local scale it appears flat. If the Universe underwent a period of cosmic inflation, then we are seeing such a small part of it that the small part we see is always going to appear flat, no matter what the overall geometry.

The magnetic monopole problem is solved by cosmic inflation in the following manner. The idea is that magnetic monopoles were created in large quantities before the period of cosmic inflation. They should still exist today, but because the Universe expanded so rapidly during cosmic inflation, their number density (how many there are per unit volume) is so tiny that we haven’t found any in the part of the Universe which we are able to observe.

The discovery made by BICEP2

Until yesterday, there had been no direct evidence of anything that cosmic inflation predicted, only agreement between the theory and things which had already been observed. One prediction of the theory is that the CMBR should be polarised in a particular way with a particular amount of polarisation (you can think of polarisation as a particular twisting of radiation, instead of vibrating in all directions it only vibrates in particular directions). The BICEP2 experiment (“Background Imaging of Cosmic Extragalactic Polarization”, the “2” indicates it is the second generation of this experiment) has been using the South Pole Telescope which is, as the name implies, at the Earth’s south pole, and has been looking for a particular signature in the CMBR – the “B-mode polarisation” as it is called.

Yesterday the team announced that they had, for the first time, detected this B-mode polarisation, which is the most direct evidence yet that the theory of cosmic inflation is correct. This polarisation comes about due to gravitational waves in the very very early Universe, so the detection of the B-mode polarisation is also direct evidence of gravitational waves, which were predicted by Einstein but have never been directly detected before.

If you want to read the actual announcement paper you can find the pre-print by following this link here. Here is a screen capture of the first page of the paper.

The first page of the paper announcing the detection of evidence for cosmic inflation.

The first page of the paper announcing the detection of evidence for cosmic inflation. Notice that Cardiff University has an involvement with Peter Ade being the first author in the alphabetical list.

Superimposed on the variations in the temperature of the cosmic microwave background (red and blue blobs) is the evidence for the B-mode polarisation (the small swirls or black lines).

Superimposed on the variations in the temperature of the cosmic microwave background (red and blue blobs) is the evidence for the B-mode polarisation (the small black swirls).

This is very exciting news for cosmology and our understanding of the earliest moments of the Universe. It suggests that our model of the early Universe, including the theory of cosmic inflation, is correct (or at least is on the right tracks). Little by little, astronomers are unfolding the mysteries of the very earliest moments of creation!

If you want to read a more technical (but still non-specialist) explanation, then this story in Sky & Telescope is pretty good. Or, you may prefer this from Sean Carroll’s blog.

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