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Tag Archives: age of the universe

Most Distant Galaxy Known: over 95% of the way back to the origin

Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.

EGSY8p7-a

Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here:

http://arxiv.org/pdf/1507.02679v2.pdf

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Scale of the universe

Hubble Ultradeep Field

Hubble Ultra Deep Field

Until 500 years ago the premise of an Earth-centric solar system and universe prevailed. And until 100 years ago it was thought that we lived within the confines of a single galaxy, our Milky Way. But in 1915 Albert Einstein introduced general relativity, the highly successful theory of gravity which couples mass, energy and the geometry of space-time. In the 1920s Alexander Friedmann and Georges Lemaitre introduced solutions to the equations of general relativity for an expanding universe. Lemaitre’s work indicated distant galaxies would have their light shifted to be redder than that of nearby galaxies. And by 1929 this was observed by Edwin Hubble. Now with the Hubble Space Telescope we can observe galaxies at much greater distances than Hubble could over 80 years ago. The image above is a very long exposure from the Hubble Space Telescope revealing close to 10,000 galaxies; many of these are billions of light-years away.

Hubble essentially measured the rate of expansion of the universe at the present epoch. The universe is expanding and galaxies are generally receding from one another except when they are gravitationally bound to their near neighbors. The value for the rate of expansion has been refined over the intervening years but is now accurately measured and indicates an age of just under 14 billion years for our universe.

The size of the universe as a whole we are unable to measure! We are limited by our own horizon, due to the finite speed of light. Only galaxies apparently moving away from us at less than the speed of light are within our horizon (also known as light cone). General relativity allows for space itself to stretch at faster than the speed of light if the separations between two galaxies are large enough; objects do not travel faster than light speed within their own local frame.

Our own observable portion of the universe has a lookback time distance of 14 billion light-years and what is known as the comoving distance of nearly 50 billion light-years. The comoving distance takes into account the expansion of the universe as the light moves through it from the Big Bang until now.

Note from the table below how much larger the universe is than the distance to the center of our galaxy or to the nearest star.

Object                    Distance (light travel time)

Nearest Star                        4.2 years
Center of Milky Way         25,000 years
Andromeda Galaxy           2.5 million years
Oldest Galaxies                  13 billion years
Big Bang                              13.8 billion years