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Topic Name: Research Team has Found New light on Mysterious Dark Energy Using ESO’s Very Large Telescope

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: International Research Team

Location: European Southern Observatory, Chile

Details

Astronomers have used European Southern Observatory’s Very Large Telescope to measure the distribution and motions of thousands of galaxies in the distant Universe. This opens fascinating perspectives to better understand what drives the acceleration of the cosmic expansion and sheds new light on the mysterious dark energy that is thought to permeate the Universe.

“Explaining why the expansion of the Universe is currently accelerating is certainly the most fascinating question in modern cosmology,” says Luigi Guzzo, lead author of a paper in this week’s issue of Nature, in which the new results are presented. “We have been able to show that large surveys that measure the positions and velocities of distant galaxies provide us with a new powerful way to solve this mystery.”

Ten years ago, astronomers made the stunning discovery that the Universe is expanding at a faster pace today than it did in the past.

“This implies that one of two very different possibilities must hold true,” explains Enzo Branchini, member of the team. “Either the Universe is filled with a mysterious dark energy which produces a repulsive force that fights the gravitational brake from all the matter present in the Universe, or, our current theory of gravitation is not correct and needs to be modified, for example by adding extra dimensions to space.”

Current observations of the expansion rate of the Universe cannot distinguish between these two options, but the international team of 51 scientists from 24 institutions found a way that could help in tackling this problem. The technique is based on a well-known phenomenon, namely the fact that the apparent motion of distant galaxies results from two effects: the global expansion of the Universe that pushes the galaxies away from each other and the gravitational attraction of matter present in the galaxies’ neighbourhood that pulls them together, creating the cosmic web of large-scale structures.

“By measuring the apparent velocities of large samples of galaxies over the last thirty years, astronomers have been able to reconstruct a three-dimensional map of the distribution of galaxies over large volumes of the Universe. This map revealed large-scale structures such as clusters of galaxies and filamentary superclusters ”, says Olivier Le Fèvre, member of the team. “But the measured velocities also contain information about the local motions of galaxies; these introduce small but significant distortions in the reconstructed maps of the Universe. We have shown that measuring this distortion at different epochs of the Universe’s history is a way to test the nature of dark energy.”

Guzzo and his collaborators have been able to measure this effect by using the VIMOS spectrograph on Melipal, one of the four 8.2-m telescopes that is part of ESO’s VLT. As part of the VIMOS-VLT Deep Survey (VVDS), of which Le Fèvre is the Principal Investigator, spectra of several thousands of galaxies in a 4-square-degree field (or 20 times the size of the full Moon) at epochs corresponding to about half the current age of the Universe (about 7 billion years ago) were obtained and analysed.

“This is the largest field ever covered homogeneously by means of spectroscopy to this depth,” says Le Fèvre. “We have now collected more than 13,000 spectra in this field and the total volume sampled by the survey is more than 25 million cubic light-years.”

The astronomers compared their result with that of the 2dFGRS survey that probed the local Universe, i.e. measures the distortion at the present time.

Within current uncertainties, the measurement of this effect provides an independent indication of the need for an unknown extra energy ingredient in the ‘cosmic soup’, supporting the simplest form of dark energy, the so-called cosmological constant, introduced originally by Albert Einstein. The large uncertainties do not yet exclude the other scenarios, though.

“We have also shown that by extending our measurements over volumes about ten times larger than the VVDS, this technique should be able to tell us whether cosmic acceleration originates from a dark energy component of exotic origin or requires a modification of the laws of gravity,” said Guzzo.

“VIMOS on the VLT would certainly be a wonderful tool to perform this future survey and help us answer this fundamental question. This strongly encourages our team to proceed with even more ambitious surveys of the distant Universe,” says Le Fèvre.

Note for Dark Energy
In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe.  Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for almost three-quarters of the total mass-energy of the universe.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. In fact contributions from scalar fields which are constant in space are usually also included in the cosmological constant. The cosmological constant is thought to arise from the vacuum energy. Scalar fields which do change in space are hard to distinguish from a cosmological constant, because the change may be extremely slow.
High-precision measurements of the expansion of the universe are required to understand how the speed of the expansion changes over time. The rate of expansion is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.
Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.
The exact nature of this dark energy is a matter of speculation. It is known to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is not very dense—roughly 10−29 grams per cubic centimeter—it is hard to imagine experiments to detect it in the laboratory. Dark energy can only have such a profound impact on the universe, making up 70% of all energy, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant.

Note for General Theory of Relativity
General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion: In other words an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.
Some of the consequences of general relativity are:
Time goes slower at lower gravitational potentials. This is called gravitational time dilation. 
Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars). 
Even rays of light (which are weightless) bend in the presence of a gravitational field. 
The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light. This does not contradict the theory of special relativity, since it is space itself that is expanding. 
Frame-dragging, in which a rotating mass "drags along" the space time around it. 
Technically, general relativity is a metric theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.

Note for Galaxy Filament
In physical cosmology, filaments are the largest known structures in the universe, thread-like structures with a typical length of 50 to 80 megaparsecs that form the boundaries between large voids in the universe. Filaments consist of gravitationally-bound galaxies; parts where a large number of galaxies are very close to each other are called superclusters.
In 2006, scientists announced the discovery of three filaments aligned to form the largest structure known to humankind, composed of densely-packed galaxies and enormous blobs of gas known as Lyman alpha blobs.

About  Very Large Telescope
The Very Large Telescope Project (VLT) is a system of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation. Each telescope has an 8.2 m aperture. The array is complemented by three movable Auxiliary Telescopes (ATs) of 1.8 m aperture. The project is organized by the European Southern Observatory.
VLT is located at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile. The VLT consists of a cluster of four large (8.2 meter diameter) telescopes, and an astronomical interferometer (VLTI) which is used to resolve fine features. The interferometer also includes a set of four 1.8 meter diameter movable telescopes dedicated to interferometric observations. The 8.2 meter telescopes have been named after the names of some astronomical objects in the local Mapuche language: Antu (The Sun), Kueyen (The Moon), Melipal (The Southern Cross), and Yepun (Venus).
The VLT 8.2 meter telescopes can be operated in three modes:
as a set of four independent telescopes (this is the primary mode of operation) 
as a single large coherent interferometric instrument (the VLT Interferometer or VLTI), for extra resolution (this is occasionally used, only for observations of relatively bright sources). 
as a single large incoherent instrument, for extra light-gathering capacity (this mode has now been abandoned, although multiple telescopes are sometimes independently pointed at the same object, either to increase the total light-gathering power, or to provide simultaneous observations with complementary instruments)

The team is composed of

L. Guzzo, A. Iovino, and O. Cucciati (INAF Osservatorio Astronomico di Brera, Merate, Italy), M. Pierleoni, J. Blaizot, G.De Lucia, and K. Dolag (Max Planck Institut für Astrophysik, Germany), B. Meneux, B. Garilli, D. Bottini, D. Maccagni, M. Scodeggio, P. Franzetti, P. Memeo, and D. Vergani (INAF IASF, Milano, Italy), E. Branchini (Universita Roma III, Italy), O. Le Fèvre, V. LeBrun, L. Tresse, C. Adami, S. Arnouts, A. Mazure, and S. de la Torre (Laboratoire d’Astrophysique de Marseille, OAMP-CNRS - Université de Provence, France), A. Pollo (Laboratoire d’Astrophysique de Marseille, OAMP-CNRS - Université de Provence, France and Andrzej Soltan (Institute for Nuclear Research, Warsaw, Poland), C. Marinoni (Centre de Physique Théorique, CNRS-Université de Provence, Marseille, France), S. Charlot (Institut d’Astrophysique de Paris, CNRS-Université de Paris 6, France), H. J. McCracken (Institut d’Astrophysique de Paris, CNRS-Université de Paris 6, and Laboratoire d'étude du rayonnement et de la matière en astrophysique, CNRS - Observatoire de Paris, France), J. P. Picat, T. Contini, R. Pellò, and E. Perez-Montero (Laboratoire d’Astrophysique de Toulouse et Tarbes, OMP-CNRS-Université de Toulouse 3, France), G. Vettolani and A. Zanichelli (INAF IRA, Bologna, Italy), R. Scaramella (INAF Osservatorio Astronomico di Roma, Italy), S. Bardelli, M. Bolzonella, A. Cappi, P. Ciliegi, F. Lamareille, R. Merighi, G. Zamorani, E. Zucca, and L. Pozzetti (INAF Osservatorio Astronomico di Bologna, Italy), A. Bongiorno and B. Marano (Universitá di Bologna, Italy), L. Moscardini (Universitá di Bologna and INFN Sezione di Bologna, Italy), S. Foucaud (University of Nottingham, UK), I. Gavignaud (Astrophysikalisches Institut Potsdam, Germany), O. Ilbert (University of Hawaii, USA), S. Paltani (Geneva Observatory and Integral Science Data Centre, Versoix, Switzerland), and M. Radovich (INAF Osservatorio Astronomico di Capodimonte, Napoli, Italy). L. Guzzo is also associated with the MPE, MPA and ESO.

In figure 1, Journey through galaxies

In figure 2, As this NASA chart indicates, roughly 70 percent or more of the universe consists of dark energy, about which we know next to nothing

In figure 3, Large-scale structures (Artist's Impression)

In figure 4, A Cone in the Universe