Observing the unseen with Euclid

Scientia — In a notorious hunt for elusive dark matter, seeing the unseen is the key to scientific success. The latest addition to the fleet of probes searching for dark matter, European Space Agency’s (ESA) Euclid spacecraft, is being designed to bring breakthrough results while observing the yet unobserved.

“Euclid will observe the unseen, meaning the part of the universe that does not emit or absorb light, but we know it is there because of the global properties of the universe, its geometrical properties and its expansion rate,” Giuseppe D. Racca, ESA’s Euclid Project Manager told Phys.org.

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To find the traces of dark matter, the spacecraft will map the large-scale structure of the universe over the entire extragalactic sky. According to ESA, the probe will measure galaxies out to redshifts of about two, looking back in time 10 billion years as it covers the period over which dark energy accelerated the expansion of the universe. Dark matter is invisible, but has gravity and acts to slow the expansion. It is now assumed that ordinary matter makes up only about 4 percent of the universe; the rest is actually dominated by dark matter and dark energy.

To achieve its ambitious goals, besides a 1.2 m-diameter telescope, Euclid will be equipped with two powerful instruments designed to carry out scientific observations.

“We have two instruments, one visible imager, called VIS, capable to resolve images of galaxies in the 550 to 900 nm passband. The VIS nominal survey images are used to determine the shapes of at least 30 galaxies per arcmin, for a total of 1.5 billion galaxies,” Racca said.

“The other instrument is NISP [an infrared instrument], designed to carry out slitless spectroscopy and imaging photometry in the near-infrared (NIR) wavelength. The NISP spectroscopy measures the redshifted H-alpha emission line of galaxies,” he added.

VIS and NISP, large-format cameras, will be used to characterize the morphometric, photometric and spectroscopic properties of galaxies. Euclid will use two techniques to observe the invisible, called galaxy clustering (GC) and weak lensing (WL).

In Euclid’s case, GC means performing a measurement of the redshift distribution of galaxies from their H-alpha emission line survey using near-infrared slitless spectroscopy. This method also provides direct information of the validity of general relativity because it enables monitoring the evolution of structures subject to the combined effects of gravity, which forces clumping of matter and the opposing force caused by the accelerated expansion.

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The WL technique measures the distortion of the galaxy shapes due to the gravitational lensing caused by the predominantly dark matter distribution from the galaxies and the observer. The obtained galaxy shear field can be transformed into the matter distribution. WL requires extremely high image quality because possible image distortions by the optical system must be suppressed or calibrated out in order to measure the true distortions caused by gravity.

“It is important that the two techniques are performed at the same time. The complementarity of the two probes will, indeed, provide important additional information on possible systematics, which limit the accuracy of each of the probes,” Racca noted.

The Euclid mission is currently on track for the planned launch in late 2020. It was selected for implementation as a Medium-class mission in ESA’s Cosmic Vision program in October 2011 and formally adopted in June 2012. The mission will operate from the Sun–Earth Lagrangian point (L2), situated 1.5 million km from Earth. Science and spacecraft operations will be conducted by ESA. The Euclid Consortium, consisting of more than one hundred scientific institutes, is responsible for the development and delivery of the spacecraft’s instruments and the scientific data processing.

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– Credit and Resource –

ESA

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Observing the unseen with Euclid

The Euclid Spacecraft

Scientia —

Euclid is an ESA medium class astronomy and astrophysics space mission. Euclid was selected by ESA in October 2011 (see the Euclid ESA page). Its launch is planned for Q1 2020. In June 2012 ESA officially selected the “Euclid Consortium” as the single team having the scientific responsibility of the mission, the data production and of the scientific instruments.

The Euclid mission aims at understanding why the expansion of the Universe is accelerating and what is the nature of the source responsible for this acceleration which physicists refer to as dark energy. Dark energy represents around 75% of the energy content of the Universe today, and together with dark matter it dominates the Universes’ matter-energy content. Both are mysterious and of unknown nature but control the past, present and future evolution of Universe.

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Euclid will explore how the Universe evolved over the past 10 billion years to address questions related to fundamental physics and cosmology on the nature and properties of dark energy, dark matter and gravity, as well as on the physics of the early universe and the initial conditions which seed the formation of cosmic structure.

The imprints of dark energy and gravity will be tracked by using two complementary cosmological probes to capture signatures of the expansion rate of the Universe and the growth of cosmic structures: Weak gravitational Lensing and Galaxy Clustering (Baryonic Acoustic Oscillations and Redshift Space Distortion).

To accomplish the Euclid mission ESA has selected Thales Alenia Space (see also the ESA press release ) for the construction of the satellite and its Service Module and Airbus Defence and Space (ex-Astrium) for the Payload Module.

Euclid will be equipped with a 1.2 m diameter Silicon Carbide (SiC) mirror telescope made by Airbus Defence and Space feeding 2 instruments, VIS and NISP, built by the Euclid Consortium : a high quality panoramic visible imager (VIS), a near infrared 3-filter (Y, J and H) photometer (NISP-P) and a slitless spectrograph (NISP-S). With these instruments physicists will probe the expansion history of the Universe and the evolution of cosmic structures by measuring the modification of shapes of galaxies induced by gravitational lensing effects of dark matter and the 3-dimension distribution of structures from spectroscopic red­shifts of galaxies and clusters of galaxies.

The satellite will be launched by a Soyuz ST-2.1B rocket and then travel to the L2 Sun-Earth Lagrangian point for a 6 years mission.

Euclid will observe 15,000 deg2 of the darkest sky that is free of contamination by light from our Galaxy and our Solar System (see the ESA Euclid mission summary ). Two “Euclid Deep Fields” covering around 20 deg2 each will be also observed extending the scientific scope of the mission the high-redshift universe.

The complete survey represents hundreds of thousands images and several tens of Petabytes of data. About 10 billion sources will be observed by Euclid out of which more than 1 billion will be used for weak lensing and several tens of million galaxy redshifts will be also measured and used for galaxy clustering. The scientific analysis and interpretation of these data is led by the scientists of the Euclid Consortium.

Euclid Scientific Objectives

Euclid is primarily a cosmology and fundamental physics mission. Its main scientific objective is to understand the source of the accelerating expansion of the Universe and discover its very nature that physicists refer to as dark energy.

Euclid will then address to the following questions:

• is dark energy merely a cosmological constant, as first discussed by Einstein, or


• is it a new kind of field that evolves dynamically with the expansion of the universe?


• alternatively, is dark energy instead a manifestation of a breakdown of General Relativity and deviations from the law of gravity?


• what are the nature and properties of dark matter?


• what are the initial conditions which seed the formation of cosmic structure?


• what will be the future of the Universe over the next ten billion years?

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The imprints of dark energy and gravity will be detected from their signatures on the expansion rate of the Universe and the growth of cosmic structures using gravitational lensing effects on galaxies (Weak Lensing) and the properties of galaxy clustering (Baryonic Acoustic Oscillations and Redshift Space Distortion). Baryon acoustic oscillations provide a direct distance-redshift probe to explore the expansion rate of the Universe. Weak lensing provides an almost direct probe of dark matter but combines together angular distances that probes the expansion rate and the mass density contrast that probe the growth rate of structure and gravity. In contrast, redshift space distortion probes the growth rate of cosmic structures and gravity. Combined together these three probes are solid and complementary probes of the effects of dark energy.

These observations will be complemented by independent observations also derived from Euclid data on clusters of galaxies and the Integrated Sachs-Wolf effect. They will be used to cross-check the results obtained from Weak Lensing, Baryonic Acoustic Oscillations and Redshift Space Distortion and to better understand and control systematic errors.

Illustration of the primary probes of the Euclid mission. Left: Baryon acoustic oscillations, (BAO), Redshift Space Distortion (RSD). Right: Weak Lensing (WL)- Courtesy Euclid Consortium/Science Working Group.

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For more information, check out the Euclid site from our awesome friends at the ESA.

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The Euclid Spacecraft

What is Dark Matter and Dark Energy?

Dark Energy and Dark Matter

In the early 1990s, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein’s theory of gravity, one that contained what was called a “cosmological constant.” Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein’s theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don’t know what the correct explanation is, but they have given the solution a name. It is called dark energy.

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What Is Dark Energy?

More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe’s expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The rest – everything on Earth, everything ever observed with all of our instruments, all normal matter – adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn’t be called “normal” matter at all, since it is such a small fraction of the Universe.

This diagram reveals changes in the rate of expansion since the universe’s birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
NASA/STSci/Ann Feild

One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein’s gravity theory, the version that contains a cosmological constant, makes a second prediction: “empty space” can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, “empty space” is actually full of temporary (“virtual”) particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong – wrong by a lot. The number came out 10120 times too big. That’s a 1 with 120 zeros after it. It’s hard to get an answer that bad. So the mystery continues.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this “quintessence,” after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don’t know what it is like, what it interacts with, or why it exists. So the mystery continues.

A last possibility is that Einstein’s theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein’s theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.

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The thing that is needed to decide between dark energy possibilities – a property of space, a new dynamic fluid, or a new theory of gravity – is more data, better data.

What is Dark Matter?

By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?

We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.

One of the most complicated and dramatic collisions between galaxy clusters ever seen is captured in this new composite image of Abell 2744. The blue shows a map of the total mass concentration (mostly dark matter).

However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or “MACHOs”. But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles)

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What is Dark Matter and Dark Energy?