STScI: Hubble Rules Out One Alternative to Dark Energy

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Hubble Rules Out One Alternative to Dark Energy
STScI | HubbleSite | 2011 Mar 14

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Hubble's View of NGC 5584
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Astronomers using NASA's Hubble Space Telescope have ruled out an alternate theory on the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy. The universe appears to be expanding at an ever increasing rate, and one explanation is that the universe is filled with a dark energy that works in the opposite way of gravity. One alternative to that hypothesis is that an enormous bubble of relatively empty space eight billion light-years across surrounds our galactic neighborhood. If we lived near the center of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion. This hypothesis has been invalidated because astronomers have refined their current understanding of the universe's present expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble's previous best measurement in 2009. The results are reported in the April 1 issue of The Astrophysical Journal.

Amongst the myriad stars in spiral galaxy NGC 5584, imaged in visible light with Hubble's Wide Field Camera 3 between January and April 2010, are pulsating stars called Cepheid variables and one recent Type Ia supernova, a special class of exploding stars. Astronomers used Cepheid variables and Type Ia supernovae as reliable distance markers to measure the universe's expansion rate. NGC 5584 lies 72 million light-years away in the constellation Virgo and was one of the eight galaxies astronomers studied to measure the universe's expansion rate. In those galaxies, astronomers analyzed more than 600 Cepheid variables, including 250 in NGC 5584. Cepheid variables pulsate at a rate matched closely by their intrinsic brightness, making them ideal for measuring distances to relatively nearby galaxies. Type Ia supernovae flare with the same brightness and are brilliant enough to be seen from relatively longer distances. Astronomers search for Type Ia supernovae in nearby galaxies containing Cepheid variables so they can compare true brightness of both types of stars. That brightness information is used to calibrate the measurement of Type Ia supernova in far-flung galaxies and calculate their distance from Earth. Once astronomers know accurate distances to galaxies near and far, they can determine the universe's expansion rate.

Credit: NASA, ESA, A. Riess (STScI/JHU), L. Macri (Texas A&M University), and Hubble Heritage Team (STScI/AURA)

Hubble’s View of NGC 5584
Hubble Heritage March 2011 Release

The brilliant, blue glow of young stars trace the graceful spiral arms of galaxy NGC 5584 in this Hubble Space Telescope image. Thin, dark dust lanes appear to be flowing from the yellowish core, where older stars reside. The reddish dots sprinkled throughout the image are most likely background galaxies.

Among the galaxy’s myriad stars are pulsating stars called Cepheid variables and one recent Type Ia supernova, a special class of exploding star. Astronomers use Cepheid variables and Type Ia supernovae as reliable distance markers to measure the universe’s expansion rate. NGC 5584 was one of eight galaxies astronomers studied to measure the universe’s expansion rate. In those galaxies, astronomers analyzed more than 600 Cepheid variables, including 250 in NGC 5584.

Cepheid variables pulsate at a rate matched closely by their intrinsic brightness, making them ideal for measuring distances to relatively nearby galaxies. Type Ia supernovae flare with the same brightness and are brilliant enough to be seen from relatively longer distances.

Astronomers search for Type Ia supernovae in nearby galaxies containing Cepheid variables so they can compare true brightness of both types of stars. They then use that information to calibrate the measurement of Type Ia supernovae in far-flung galaxies and calculate their distance from Earth. Once astronomers know accurate distances to galaxies near and far, they can determine the universe’s expansion rate.

The image is a composite of several exposures taken in visible light between January and April 2010 with Hubble’s Wide Field Camera 3.

NGC 5584 resides 72 million light-years away in the constellation Virgo.

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Dark energy is not an illusion after all
New Scientist | David Shiga | 2011 Mar 16

New measurements of exploding stars are challenging an upstart theory that dark energy is just an illusion caused by our location within a giant void.

In 1998, astronomers reported that the universe's expansion seems to be faster now than it was in the past, based on measurements of supernova explosions in both nearby and distant galaxies. The latter provide a record of the past because of the time it takes their light to reach us.

That the universe's expansion could be accelerating was a surprise, since gravity should act as a brake on the expansion, slowing it with time. The most popular explanation is that energy of unknown origin – called dark energy – permeates space and acts as a repulsive force to speed up the expansion.

But some researchers have proposed an alternative: that the acceleration is an illusion that results from an uneven distribution of matter in the universe.

Dark pedal

They accept that the expansion rate in the local universe is higher than in more distant regions. But instead of assuming the expansion rate has increased with time, they suggest our patch of the universe happens to contain less matter than average. Within this "void", the expansion rate is higher than outside because there is less gravity to slow it down.

But new, more precise measurements of supernovae, taken by the Hubble Space Telescope, clash with the simplest version of the void model. That model could be made to fit previous supernova measurements and other cosmological data, but only if the local expansion rate is about 60 kilometres per second per megaparsec or less. (One megaparsec is 3.26 million light years.)

That was within the possible error of previous measurements, but the new, more precise measurements give an expansion rate of 74 kilometres per second per megaparsec, plus or minus 2.4.

"It looks more like it's dark energy that's pressing the gas pedal," says Adam Riess of Johns Hopkins University in Baltimore, Maryland, who led the observations. The results appear in The Astrophysical Journal.

Void within a void?

But Subir Sarkar of the University of Oxford, a proponent of the void theory who was not involved in Riess's study, says the results are not a fatal blow. "The observers have done a good job, but it should be kept in mind that there is some flexibility in the alternative models, which can in fact accommodate higher values" for the local expansion, he says.

He points to a study by Tirthabir Biswas of Saint Cloud State University in Minnesota and colleagues, published in November in the Journal of Cosmology and Astroparticle Physics, which tested a variety of void models against astronomical data. Some of them allow local expansion rates as high as the new Hubble value by positing a "void within a void", where the density of matter is not constant within the void itself, but drops off steeply towards its centre.

Although such a model might seem contrived, the alternative is to invoke dark energy, whose origin is very hard to explain, says Sarkar. "I would rather believe that the universe is a little more complicated than the standard cosmological model assumes it to be," he says.

A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3 - AG Riess et al

• Astrophysical Journal 730(2) 119 (2011 Apr 01) DOI: 10.1088/0004-637X/730/2/119
  arXiv.org > astro-ph > arXiv:1103.2976 > 15 Mar 2011

Testing the void against cosmological data: fitting CMB, BAO, SN and H₀ - T Biswas et al

• Journal of Cosmology and Astroparticle Physics 2010(11) 030 (2010 Nov 22) DOI: 10.1088/1475-7516/2010/11/030
  arXiv.org > astro-ph > arXiv:1007.3065 > 19 Jul 2010 (v1), 05 Nov 2010 (v2)

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NGC 5584: Spiral Key to Universe's Expansion
ESA/HEIC Hubble Picture of the Week | 2011 Mar 21

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This view from the NASA/ESA Hubble Space Telescope shows the beautiful spiral galaxy NGC 5584. This galaxy has played a key role in a new study that measures the expansion rate of the Universe to greater accuracy than ever before.

NGC 5584 was first spotted as a faint glow in the constellation of Virgo by the great visual observer E. E. Barnard, back in 1881, using just a 12.5-cm telescope. But, by bringing the power of Hubble to bear, the galaxy can be resolved into thousands of separate stars. Some of these stars vary in brightness and are classified as Cepheids. These are brilliant pulsating stars with a remarkable property — once the time it takes a Cepheid to brighten and fade is known, then it is possible to find how bright it actually is. When this information is combined with a measurement of how bright the star appears it is easy to work out how far away the star actually lies. This method is the most accurate and effective way to measure the distances to most nearby galaxies.

This trick has now been used as part of a major new study of the expansion rate of the Universe, led by Adam Riess at the Space Telescope Science Institute in Baltimore. By studying many Cepheids in several galaxies the team has been able to refine our knowledge of this expansion rate, expressed as a number known as Hubble’s constant, to an accuracy of 3.3 percent.

In addition to many Cepheids NGC 5584 was also recently the site of a type Ia supernova. These dramatic explosions of white dwarf stars are used as reference beacons for mapping the expansion, and acceleration, of the more remote Universe so this galaxy is a very valuable link between the two distance scales.

More details of this major study, and its significance for the understanding of dark energy, can be found in a press release from NASA.

This picture was created from many exposures taken with Hubble’s Wide Field Channel 3. Images through three filters have been combined to create this composite picture. Light detected through a filter that transmits most visible light (F350LP) is coloured white, light coming through a yellow/green filter (F555W) is coloured blue and near infrared light (the F814W filter) is coloured red. The field of view 2.4 arcminutes across and the total exposure time was 20.8 hours.

Credit: NASA, ESA, A. Riess (STScI/JHU), L. Macri (Texas A&M), and the Hubble Heritage Team (STScI/AURA)

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