Cosmology

According to the standard cosmological model, the Universe started with a Big Bang about 14 billion years ago. During an early epoch of accelerated superluminal expansion, called inflation, a region of microscopic size stretched to a scale much larger than the visible Universe and our local geometry became flat. At the same time, quantum mechanical fluctuations of the vacuum generated primordial density fluctuations in the matter distribution. Gravity enhanced these inhomogeneities, seeding the formation of present-day structure.

The mass density of ordinary (baryonic) matter makes up only a fifth of the matter that led to the emergence of structure. The rest is in the form of an unknown dark matter component. Recently, the Universe entered a new phase of accelerated expansion due to the dominance of some dark vacuum energy density over the ever-lower matter density. This "dark energy" accounts for more than 70% of the mass-energy density of the Universe.

Extra Galactic Surveys

Viewed in X-rays the night sky glows brightly and uniformly in all directions. The origin of this 'cosmic X-ray background' was one of two great puzzles set by the very first observations of X-rays from beyond the Solar System by Riccardo Giacconi and his team back in 1963. (For which Giacconi won the Nobel Prize in Physics in 2002). To solve the puzzle required sensitive X-ray telescopes, culminating in the Chandra X-Ray Observatory.  This X-ray glow is due to the summed emission from millions of faint quasars and active galactic nuclei that can now be seen individually in deep Chandra images.
To discover this answer required surveys of the sky away from the confusing and absorbing plane of the Milky Way, so objects in our Galaxy contribute little leading these to be called 'Extragalactic Surveys'. Even though we not know the answer, HEA (Harvard Center for Astrophysic studies) scientists continue to make new extragalactic surveys to learn more about how quasars evolve over the history of the universe, and to study even fainter sources - distant galaxies and, eventually, the first stars, galaxies and black holes to form in the universe.

This virtual journey, from the centre of the Milky Way to its outskirts, shows the different components that make up our Galaxy, which is home to about a hundred billion stars.

With a mass of four million Suns, a supermassive black hole (known as Sagittarius A*) sits at the centre of the Galaxy, its enormous gravity governing the orbits of stars in its vicinity. Stars have been observed orbiting this black hole at distances as close as a few light-days.

Moving outwards, we fly through a multitude of stars of the Galactic Bulge. The bulge is  located in the central portion of the Milky Way and hosts about ten billion stars, which are mainly old and red. The bulge has an overall elongated shape that resembles that of a peanut-shaped bar, with a half-length of about 10 000 light-year, making the Milky Way a barred spiral galaxy.

Beyond the bulge, the journey continues across the Galaxy, flying through the younger population of stars in the stellar disc. Home to most of the Milky Way's stars, the stellar disc is a flattened structure with a radius of about 50 000 light-years and a vertical height of only 1000 light-years. The stellar disc is embedded in a slightly larger structure, the gaseous disc. Stars in the disc are arranged in a spiral arm pattern and orbit the centre of the Galaxy.

The discs and bulge are embedded in the stellar halo – a spherical structure which consists of a large number of globular clusters – the oldest population of stars in the Galaxy –  as well as many isolated stars.  The stellar halo extends out to a radius of about 100 000 light-years.

Astronomers believe that, like most galaxies, the Milky Way is embedded in an even larger halo of invisible dark matter. Since it does not emit any light, the presence of the dark matter halo can only be inferred indirectly by its gravitational effect on the motions of stars in the Galaxy.

Having seen our Galaxy from afar, we zoom into the disc again and change viewing direction, revealing a face-on view of the  spiral arm structure of the Milky Way. The position of the Sun, located at a distance of about 26 000 light-years from the Galactic Centre – roughly half way between the centre and the outskirts of the Milky Way – is shown.

Finally, an indication is given of the size of the survey of stellar distances performed by ESA's Hipparcos mission, which operated between 1989 and 1993. The Hipparcos catalogue, published in 1997, contains the position, proper motion and distance of more than 100 000 stars up to 300 light-years away from the Sun. The survey performed by ESA's Gaia mission will probe one billion stars, about one per cent of the total number of stars in our Galaxy, out to 30 000 light-years away – one hundred times farther than Hipparcos.