Get ready for September 14, 2016. On that date, the European Space Agency's Gaia probe will offer us its first map of the Milky Way. The probe will give us answers to important questions about our galaxy and our universe in general. The history of the Milky Way, the location of dark matter, the expansion of the universe and even thousands of exoplanets.
Gaia, a space telescope launched by the European Space Agency in 2013, will publish its first map of the Milky Way on 14 September 2016. This catalogue contains the 3D positions of 2 057 050 stars and other objects and how these positions have changed over the last two decades. Over time, the map will contain 1 billion objects and will be ten times more accurate than any map we have today.
Next week's publication will also include 19 papers by Gaia astronomers who have already seen the data. But independent teams are also ready to evaluate them. Lennart Lindegren, an astronomer at the Lund Observatory in Sweden and one of the main promoters of the Gaia project since 1993, hopes that the astronomers will produce more than 100 papers in the weeks following the publication of the map.
Some groups are even considering Gaia Hacking or Gaia Sprint type events where researchers will coordinate to exploit this sudden wealth of data. Gaia will revolutionize our knowledge of stars and the Milky Way according to David Hogg, a New York University astronomer who will lead some of these events. But what can we expect from the Gaia probe?
The archaeology of the Milky Way
The Gaia probe will help us better understand the structure of the Milky Way...
Gaia's 3D vision of the Milky Way will reveal how stars move under its combined gravitational force. This will help to understand the structure of the galaxy including parts that are not visible from Earth such as the Bar, two arms that run from the galactic center to join her spiral arms.
Researchers will also be able to identify particular groups of stars that travel to large cities and are believed to be the remnants of mergers with small galaxies, according to Michael Perryman, an astronomer at University College and a former ESA Gaia official. Combined with existing information such as the colour, temperature and chemical composition of the stars, this map will allow researchers to reconstruct the archaeology of the galaxy. Throughout its existence, Gaia will revolutionise our understanding of the Milky Way according to Monica Valluri, an astronomer at the University of Michigan.
Where is the dark matter of the Milky Way?
The details of the stars' trajectories will reveal not only visible matter, but also dark matter. And this will allow us to discover where that dark matter is. Gaia will also allow us to test exotic hypotheses. The Standard Dark Matter Hypothesis predicts that the gravitational force of the Galaxy is spherically symmetrical next to the center, but becomes elongated like an American football according to Valluri. But an alternative hypothesis called WORLD (Modified Newtonian dynamics) implies that this pancake-like gravitational force. By analyzing the velocity of stars, which depends on the gravitational force, Gaia will help confirm or disprove one of these hypotheses.
We can go further. Maybe Gaia's data will tell us if it's the dark matter that killed the dinosaurs. If the dark matter is concentrated in a black disc shape next to the galactic plane according to this bold hypothesis, then it may have triggered the asteroid impacts that caused mass extinctions in the solar system.
The debate on stellar distances
The Pleiades Cluster, Gaia will give us its exact distance from the sun...
Accurate measurements of the distance of stars from our sun will allow astrophysicists to refine the model of the evolution of stars. Our current theories rely heavily on distance estimates to understand how the intrinsic light of a star changes during its existence.
One of the first groups of stars targeted by researchers will be the Pleiades, a cluster in the constellation Taurus. Most observations, including the Hubble Telescope, place the cluster at about 135 Parsecs (440 light years). But results based on data from Hipparcosa space mission that preceded Gaia, suggest that this cluster is only 120 parsecs away.
Some people doubt the measure of Hipparcos. Gaia will use a similar instrument, but much more powerful. I think that Hipparcos' results will be invalidated by Gaia according to David Soderblom of the Space TelescopeScience Institute in Baltimore.
Thousands of new worlds
Thousands of new exoplanets thanks to the Gaia probe...
Astronomers have discovered thousands of planets orbiting other stars. Typically, they are discovered by detecting the decrease in brightness of the star as the planet passes in front of it. Gaia will detect exoplanets by another method. It measures the slight oscillations in the star's position that are caused by the planet's gravitational force.
"I bet Gaia will discover a thousand new worlds." says Gregory Laughlin, an astronomer from Yale University. Gaia's technique is suitable for detecting large planets with wide orbits according to Alessandro Sozzetti, a Gaia researcher at the Astrophysical Observatory in Turin, Italy. And unlike the method on luminosity decrease, Gaia's technique directly measures the mass of planets. But the search for exoplanets with this method will take years and we won't have the results by 2018.
How fast is the expansion of the universe accelerating?
The star RS Puppis, used as a cosmic candle to measure the expansion of the universe
Even though Gaia is an explorer of the Milky Way, her influence reaches all of the observable universe. Gaia's direct distance measurements work only for the Milky Way and its vicinity. To estimate the distances of distant galaxies, astronomers wait for explosions such as Supernovæ Ia. The brightness of such a supernova reveals the distance to the corresponding galaxy. These celestial beacons, known as cosmic candlesticks, have been the main tools for measuring the speed of the expansion of the universe. These measurements have led astronomers to propose a mysterious dark energy that is behind this expansion.
But to use supernovas as beacons, astronomers have to compare them with other standard candles in our galaxy. In the first map, Gaia will measure the distances of thousands of these stars with great accuracy. Later, the probe's measurements will allow cosmologists to improve the map of the entire universe and resolve conflicting estimates of the expansion of the universe.
The invisible asteroid threat...
As she constantly scans the sky, Gaia will also discover things close to us. The goal is to detect 350 000 asteroids inside the solar system, according to Paolo Tanga, Gaia's astronomer at the Cote d'Azur observatory in Nice. And these will include near-Earth objects whose orbits bring them to a distance of 200 million kilometres from Earth.
If Gaia detects a near-Earth object, then the probe can alert ground-based observatories to determine if the object is a threat. From its vantage point in space, Gaia will scan the entire sky to reveal objects that are too close to the sun to be detected from Earth, according to Anthony Brown, an astronomer at the Leiden Observatory in the Netherlands who chairs the collaboration on Gaia's data analysis. And by tracking certain asteroids that orbit the sun over several years, Gaia will be able to perform very sensitive tests of the theory of general relativity.