Archive for ‘Cosmology’

November 23, 2011

Most stars in dwarf galaxies formed early in the universe

A study based on a survey of very distant galaxies selected 69 individual objects that had exceptionally bright emissions lines, suggesting that they were galaxies very actively forming new stars. Out of that number, four objects for which the necessary measurements could be made proved to have strong lines due to ionized oxygen. In these cases it was possible to conclude that the objects were relatively small galaxies whose mass was about 100 million solar masses (M) – and they were forming stars so rapidly that the mass due to stars would double in just 10 million years – an average of 10 M per year. That compares to a rate of about 3 M per year at present in the Milky Way – even though it is 1000 times as massive as the early galaxies studied.

The galaxies studied were at a redshift z~1.7, which corresponds to a light-travel distance of 11.5 billion light-years, so the galaxies are seen as they were about 2.2 billion years after the big bang. Other surveys have suggested that this era was when the rate of star formation everywhere was near its peak. From the observed numbers of small galaxies of ~108 M and the star formation rates at z~1.7 it can be estimated that most of the stars in present-day dwarf galaxies (from 108 M to 109 M) formed in a few short bursts from 1 to 5 billion years after the big bang.

Simulations of star formation in dwarf galaxies suggests that rapid star formation activity occurs in bursts. This is because when star formation occurs most rapidly the combined energy emitted by the hot young stars quickly either heats gas inside the galaxy to a temperature too high for new stars to form or expels the gas entirely. Only after enough time has passed following the burst does the gas cool off enough and fall back into the galaxy to restart the cycle. So the 4 galaxies most carefully studied are probably not representative, since they were more likely to have been noticed than similar galaxies not in the middle of a burst of star formation.

According to the research paper:

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

Our discovery of an abundant galaxy population at z ~ 1.7 with extremely high emission line equivalent widths implies that many high-redshift, low-mass galaxies form many of their stars in extreme starbursts. We propose that we have observed an important formation mode for dwarf galaxies: a small number of strong starbursts that occur at early epochs (z > 1) each form ~108 M in stars in a very short time span (~30 Myr) to build up the bulk of the stellar components of present-day dwarf galaxies. This is in quantitative agreement with ’archaeological’ studies of present-day dwarf galaxies, which have shown that their star formation histories are burst-like and that the ages of their stellar populations suggest formation redshifts z > 1. Under the reasonable assumption based on ΛCDM predictions for galaxy growth that the observed galaxies grow in mass by less than an order of magnitude up to the present day, our observations provide direct evidence for such an early formation epoch and, in particular, that short-lived bursts contribute much or even the majority of star formation in dwarf galaxies.

Further reading:

Hubble Uncovers Tiny Galaxies Bursting with Starbirth in Early Universe

Population explosion in dwarf galaxies

Tiny galaxies bursting with stars

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

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October 9, 2011

How to spot a multiverse

According to Kurt Vonnegut, “The universe is a big place, perhaps the biggest.”

However, it may not be the only place, nor even the biggest.

How to spot a multiverse – physicsworld.com

How can we tell if another universe has collided with our own? Physicists in Canada and the US believe they have the answer – it would leave “a unique and highly characteristic” imprint in the microwave background that pervades the cosmos. The physicists claim that the prediction can be tested using existing and future space telescopes, which contradicts a widespread view that the existence of a multiverse is untestable.

Chuck Bennett, an astrophysicist at Johns Hopkins University in Maryland, US, who was not involved with the study, believes the prediction helps bring multiverse theory into the realms of conventional, falsifiable science. “Science relies on being able to falsify ideas through experiment or observations of nature,” he says. “The fact that these potentialities exist enables us to call this ‘science’. That, to me, is a significant statement.”

The possibility of a multiverse comes from both string theory and inflation theory, the idea that our universe underwent a rapid expansion just after the Big Bang. Inflation theory does a good job of explaining why space is fairly smooth on large scales, but researchers can’t explain what started the expansion and what stopped it. These problems have led physicists to consider the possibility that inflation could occur at other places and times, generating new universes in addition to our own.

In order for a scientific hypothesis to be more than just theoretical speculation, there need to be testable predictions. Most people have assumed that even if other universes exist in a multiverse, there would be no way that we could detect evidence of their existence. However, that’s a speculative assumption too.

Three researchers have now suggested where and how to look for evidence.

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