April 9, 2013

Euclid Mission Studies Galaxies

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Prof. Rachel Bean, astronomy, was one of the 40 U.S. researchers chosen to be part of the Euclid mission, which will attempt to map the geometry of the dark universe. The mission, led by the European Space Agency, was named in honor of the Greek mathematician, Euclid who is considered the father of geometry.

According to Bean, the idea behind this mission is to complete a census of the galaxies in the universe dating back to the time when the universe was one-third the size it is today.

Such a census is performed by studying the light emitted by these galaxies. Some of the galaxies emitted light a long time ago, but it takes a significant amount of time to reach Earth due to the great distances between galaxies.

Studying these light signals gives researchers like Bean a snapshot of the universe when the light was emitted.

The Euclid mission is looking to pick up light signals from galaxies both near and far and will be working primarily in the optical and near-infrared wavelengths of light. For this purpose, a telescope 1.2 meters in diameter is scheduled to be launched in 2020.

Some of the key questions this mission seeks to answer are about the number and distribution of galaxies in the universe, the evolution of the universe in the past few billion years and the creation of the universe.

The researchers involved in this mission will study the optical signals to estimate the position of the galaxies.

Light stretches as it travels, since the universe is constantly expanding. This stretching leads to a phenomenon called “redshift” in which the wavelengths shift to the longer wavelength region of the spectrum, or the red side. The longer the light travels, the more the light stretches. By studying the amount of redshift in the light signals from a galaxy, we can estimate how far the light has travelled and hence the position of the galaxy.

Researchers are looking to use two different techniques for this purpose. The first one, photometry, involves studying the color of the light emitted by galaxies. The amount of light emitted by the galaxies in the “redder” band of wavelengths will be compared to the amount of light in the “bluer” band of wavelengths to estimate the amount of redshift.

The second technique to be employed is spectroscopy, which involves looking at the spectra of specific atoms. Each atom or molecule emits light at specific wavelengths, both in the lab or anywhere in the universe. But due to the redshift of the light as it travels across the universe, the wavelength of the light detected from distant atoms will be larger than that of the wavelength from atoms measured in the lab. By measuring the difference in wavelengths in the spectra, the amount of redshift can be estimated.

Most research uses only one of these techniques, but the Euclid mission will incorporate both, since the former method will provide a quicker estimate of the position of the galaxies, enabling the study of more galaxies, while the latter method provides a more precise measurement.

The cosmos is not only made of visible stars, however, but also of dark matter and dark energy. Dark matter and dark energy are estimated to comprise 95  percent of all the mass and energy in the universe. Dark matter is invisible matter which does not interact with light but is known to be affected by gravity.

Evidence for the existence of dark matter comes from studying the rotation of stars around galaxies: these stars rotate faster than expected, hinting at the existence of dark matter.

Dark energy, on the other hand, can be thought of as an anti-gravity phenomenon, the evidence for which comes from the fact that the universe is expanding at an accelerated rate, which is contrary to Einstein’s postulation that the universe should be contracting due to gravitational effects. This missing piece of physics was coined “dark energy.”

To study dark matter and energy, the Euclid mission will map the properties of gravity in the cosmos by looking at how the passage of light is distorted due to gravity as it passes through the cosmos towards Earth. This technique is called gravitational lensing and is sensitive to all matter and energy in the universe, including dark matter and dark energy.

Bean will be co-leading one of the tasks of the Euclid mission by applying this gravity survey to test gravity on cosmic scales.

Bean will use Cornell’s institutional membership at a ground-based Large Aperture Synoptic Telescope (LSST) facility located in Chile to survey half the sky and provide measurements of the cosmos that will be complementary to the Euclid mission.

Bean will also lead a group of scientists in the LSST Dark Energy Science collaboration, which works on how to test fundamental theories with observations. In particular, she will be working on translating theories that motivate dark energy, such as the particle physics model, to what would be expected in galactic observations.

“Astronomy, at the moment, is seeing a renaissance in the sheer amount of data that is coming in and the ties between theories and observations. Observations are really driving how the theories are developing, and this is an extraordinarily exciting time for students interested in this area to get involved,” Bean said.

Original Author: Srinitya Arasanipalai

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