New X-ray Astronomy Satellite ASTRO-H Striving to Solve the Mysteries of the Universe Probing the Evolution of the Universe with Galaxy Clusters Tetsu Kitayama Associate Professor, Department of Physics, Faculty of Science, Toho University

Universe began with ultra-high temperatures

Q. What is your research specialty?

My specialty is observational cosmology. Cosmology in general is the study of how the universe began, how it has evolved, and how it will develop in the future; observational cosmology deals with questions that can be verified through observations. The main observational tools are electromagnetic waves – light in the broad sense – such as optical and radio waves and X-rays, by which we can study various celestial objects and matter in space.
It takes a long time for electromagnetic waves to travel though space, so looking into distant corners of the universe means seeing its history. We think the universe was born about 14 billion years ago – that event is known as the Big Bang – but we cannot see the very early period after its birth simply with the help of electromagnetic waves; the earliest we can see is about 380,000 years after the Big Bang. We cannot observe the earlier period directly because the density and temperature of the universe were too high for electromagnetic waves to pass through. Thus, we are forced to speculate from a combination of various theories (in the future, we may be able to make observations using gravitational waves and neutrinos).
To sum up, my specialty is the study of the evolution of the universe mainly after 380,000 years from its birth, based on observational data. I am specifically interested in the formation of galaxies and galaxy clusters.

Q. How do scientists think the universe was born and developed?

A map of temperature distribution across the entire sky, reflecting density fluctuations of the early universe, observed by the microwave observation satellite WMAP. Red indicates a higher-temperature area and blue a lower-temperature area. (courtesy: NASA/WMAP Science Team)
A map of temperature distribution across the entire sky, reflecting density fluctuations of the early universe, observed by the microwave observation satellite WMAP. Red indicates a higher-temperature area and blue a lower-temperature area. (courtesy: NASA/WMAP Science Team)
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The universe started at high temperature and high density – that’s the so-called Big Bang. From that point, it has been expanding and cooling down in the course of which various celestial objects, including our Galaxy and the solar system have appeared.
Celestial objects have been created because there is a lack of balance in the density of matter in the universe; in other words, particles such as protons and electrons are unevenly distributed through space. This is called “density fluctuation”. As the gravity of a high-density region is stronger than its surroundings, it attracts more matter toward itself, thereby increasing its density and gravity even further and attracting yet more matter. That’s how smaller clumps of matter gradually becomes bigger, and galaxies and galaxy clusters are born. Tiny density fluctuations are in fact observed to be in place 380,000 years after the Big Bang.
We now have an approximate understanding of the above scenario, but there still remain many unknowns regarding the detailed properties of celestial objects. The early life of the universe, which cannot be directly observed, is also shrouded in mystery.

Dark matter comprises a major part of the mass of galaxy clusters

Q. What kind of celestial object is a galaxy cluster?

A colliding galaxy cluster 1E 0657-56. Red shows the distribution of high-temperature gas observed with X-rays, and blue indicates the distribution of dark matter, detected by the gravitational lensing effect. (courtesy: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)A colliding galaxy cluster 1E 0657-56. Red shows the distribution of high-temperature gas observed with X-rays, and blue indicates the distribution of dark matter, detected by the gravitational lensing effect. (courtesy: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)
Schematic drawing of the gravitational lensing effect. Due to the gravitational pull of dark matter in a galaxy cluster, the trajectory of light from a galaxy behind that cluster is bent, giving rise to multiple images of the same galaxy. By finding out how much light is affected, we can measure the distribution of dark matter. (courtesy: NASA/CXC/M.Weiss)
Schematic drawing of the gravitational lensing effect. Due to the gravitational pull of dark matter in a galaxy cluster, the trajectory of light from a galaxy behind that cluster is bent, giving rise to multiple images of the same galaxy. By finding out how much light is affected, we can measure the distribution of dark matter. (courtesy: NASA/CXC/M.Weiss)
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A galaxy is a collection of many stars, and a galaxy cluster is made up of anywhere from tens to thousands of galaxies. A galaxy cluster is the largest object in the universe, and by studying how these clusters are distributed and how they evolve, we can find clues to the mystery of the evolution of the entire universe.
Note that a galaxy cluster is not just a collection of galaxies. In reality, all of the galaxies in it together comprise only a few percent of its total mass. Of the remaining mass, about 15 percent is high-temperature gas at more than 10 million degrees Kelvin, and another 80 percent is dark matter.

Q. What is dark matter?

Dark matter is an unknown substance whose mass and gravity influence its surrounding area, but that doesn’t emit any kind of electromagnetic waves such as optical light or X-rays. There are many theories about dark matter, but currently its true identity remains uncertain.

Q. How do we know about the existence of dark matter?

You cannot directly detect dark matter with electromagnetic waves, but there are at least three evidences that support the gravitational effects of dark matter in a galaxy cluster.
First of all, galaxies within a cluster are not still; rather they are moving at speeds ranging from several hundred to more than 1,000 kilometers per second.
Observations reveal that these movements are influenced strongly by the gravity of dark matter. This is almost the same principle as the fact that the speed of the Earth’s orbital movement is affected by the gravity of the Sun.
Second is the presence of high-temperature gas, which I mentioned before. If dark matter didn’t exist, high-temperature gas would diffuse out of the galaxy cluster and vanish. Instead, the gas stays inside the cluster. So we understand that it’s held there by the gravitational pull of dark matter.
The third evidence is a phenomenon called gravitational lensing effect. This is a phenomenon in which the light of celestial objects coming from the other side of a galaxy cluster curves, as if it is passing through a lens. This effect is also attributed to the gravity of dark matter.

Using X-rays to study the formation of galaxy clusters

Q. Why do galaxy clusters have high-temperature gas?

A huge galaxy at the center of the galaxy cluster PKS 0745-191, captured by the Hubble Space Telescope. (courtesy: NASA/STScl/Fabian, et al.)
A huge galaxy at the center of the galaxy cluster PKS 0745-191, captured by the Hubble Space Telescope. (courtesy: NASA/STScl/Fabian, et al.)
X-ray emissions from high-temperature gas as far as 5 million light years from the center of the galaxy cluster PKS 0745-191, captured by the X-ray astronomy satellite Suzaku. The square in the center indicates the area of the image above. (courtesy: NASA/ISAS/Suzaku/M.George, et al.)
X-ray emissions from high-temperature gas as far as 5 million light years from the center of the galaxy cluster PKS 0745-191, captured by the X-ray astronomy satellite Suzaku. The square in the center indicates the area of the image above. (courtesy: NASA/ISAS/Suzaku/M.George, et al.)

Because a galaxy cluster has huge mass. The greater the mass, the stronger the gravity. When gas particles in space are attracted by gravity, they accelerate and heat up. The greater the speed, the higher the temperature. The gas’s temperature indicates the average speed of its particles (mainly protons and electrons in a galaxy cluster).

Q. How many galaxy clusters are there in space?

So far, tens of thousands of galaxy clusters have been found. As observation becomes more advanced, I think that number will grow further.

Q. Are X-rays effective in observing galaxy clusters?

Yes. Normal matter – as opposed to dark matter – emits various kinds of electromagnetic waves, depending on its temperature. For example, our bodies release infrared rays at an absolute temperature of about 300 degrees Kelvin. The surface of the Sun emits optical light at about 6,000 degrees Kelvin. The high-temperature gas in a galaxy cluster emits a huge amount of X-rays, which shine at a temperature of more than 10 million degrees Kelvin. Most of the matter in a galaxy cluster is dark matter, but unfortunately we cannot see it directly. So we use X-rays to observe the cluster’s second-most dominant element – high-temperature gas. X-ray data give us various kinds of information, including the density, temperature and composition of high-temperature gas. They provide an important clue to the formation of a galaxy cluster.

Q. What role have X-ray astronomy satellites played in the study of the evolution of the universe?

Through X-ray observations we can mainly observe the period starting about a billion years after the birth of the universe – the time when various celestial objects started to appear. Some of these celestial objects have imprints of the evolution of the universe, so studying their properties helps us understand that evolution. This is very similar to studying fossils to learn the history of the Earth. For example, from X-ray observations of a galaxy cluster we have learned such things as how much density fluctuations have grown by now, as well as how much matter there is in the universe in total.

  
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