Cosmic timeline 15
The age of the Milky Way
In 2007, a star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years, nearly as old as the Universe. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way.
Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters. Dark matter is part of science’s current theory about how the universe is made up of three different components – normal matter, which is the physical objects in the universe such as the planets – dark matter, which is invisible matter that creates the gravitational pull that causes galaxies to form – and an unknown energy referred to as “dark energy”, the force which causes the universe to expand.
Dark matter as a theory comes into play to explain how stars revolve around the center of galaxies at a constant speed over a large range of distances from the center of the galaxy. They revolve much faster than would be expected. This problem is called the galaxy rotation problem,this difference between the observed rotation speeds of matter in the disk portions of spiral galaxies and the predictions of Sir Isaac Newton‘s gravitational theory.
This discrepancy is currently thought to betray the presence of dark matter that permeates the galaxy and extends into the galaxy’s halo.
In the period when galaxies were forming cosmologists have this theory about what they call dark energy, as the possible explanation of how and why the universe is expanding and continues to expand. So, dark energy is the most popular way to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe. The simplest explanation for dark energy is that it is simply the “cost of having space”: that is, a volume of space has some intrinsic, fundamental energy. This is the cosmological constant, sometimes called Lambda (hence Lambda-CDM model) after the Greek letter Λ, the symbol used to mathematically represent this quantity.
Since energy and mass are related by E = mc2, Einstein’s theory of general relativity predicts that it will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty vacuum.
Einstein was right
The Daily Telegraph reported on 25.03.2010
In the biggest survey ever conducted using the Hubble Space telescope, 446,000 galaxies were studied to see how matter was distributed throughout the universe and how quickly it had expanded. And the astronomers found that the universe was growing faster and faster with time, as predicted by Einstein in his theory of general relativity.
Einstein’s theory of general relativity claims that space and time are a geometrical structure which can be changed by the behaviour of the matter inside it. So proof that the expansion of the universe is speeding up shows that the contents of the universe, such as the “dark energy” causing it to inflate, are influencing its structure.
Ludovic Van Waerbeke, of the Department of Physics and Astronomy at Leiden University in the Netherlands, said: “Our results confirmed that there is an unknown source of energy in the universe which is causing the cosmic expansion to speed up, stretching the dark matter further apart exactly as predicted by Einstein’s theory.
“The data from our study are consistent with these predictions and show no deviation from Einstein’s theories.”
Mr Van Waerbeke pioneered a technique to measure the invisible web of dark matter which was used in the study. In a method similar to taking an X-ray of the body to reveal the underlying skeleton, the technique, known as weak gravitational lensing, allows astronomers to see how light from distant galaxies is bent and distorted by the dark matter as it travels towards earth.
They can then map the dark matter structures, which make up 80 per cent of the universe. The study leader Tim Schrabback said: “What we tested is how the structure of the universe grows with time. If the universe expands then the gravitational lensing changes because the distance between the objects has changed.
“I think from that perspective this is quite exciting because this is the first time this measurement has been done with weak gravitational lensing alone. Before it’s always been done with other measurements because the lensing was not effective.
“The other thing that is really exciting is that in the next years there will be much larger surveys which will have more accurate measurements for dark energy and expansions, and we will see some very interesting results.”
As well as this technique, astronomers in the study used the Cosmic Evolution Survey (COSMOS), a joint project by the European Space Agency and NASA involving over 100 scientists from a dozen countries.
For the survey, a camera aboard the Hubble telescope photographed 575 slightly overlapping views of the same part of the universe, which took nearly 1,000 hours of observation, during which the Hubble circled the earth nearly 600 hundred times.
Mr Schrabback said: “This is the largest survey ever done with the Hubble telescope. The images make up a large mosaic of the sky.
“The Hubble has a great advantage from images taken from earth because there is no blurring from the atmosphere.”
According to the NASA‘s Goddard Space Flight Center Scientists Detect Cosmic ‘Dark Flow’ Across Billions of Light Years 09.23.08
Using data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), scientists have identified an unexpected motion in distant galaxy clusters. The cause, they suggest, is the gravitational attraction of matter that lies beyond the observable universe.
“The clusters show a small but measurable velocity that is independent of the universe’s expansion and does not change as distances increase,” says lead researcher Alexander Kashlinsky at NASA’s Goddard Space Flight Center in Greenbelt, Md. “We never expected to find anything like this.”
Kashlinsky calls this collective motion a “dark flow” in the vein of more familiar cosmological mysteries: dark energy and dark matter. “The distribution of matter in the observed universe cannot account for this motion,” he says.
Galaxy clusters like 1E 0657-56 (inset) seem to be drifting toward a 20-degree-wide patch of sky (ellipse) between the constellations of Centaurus and Vela.
The astronomers teamed up with Dale Kocevski at the University of California, Davis, and Harald Ebeling from the University of Hawaii to identify some 700 X-ray clusters that could be used to find the subtle spectral shift. This sample includes objects up to 6 billion light-years — or nearly half of the observable universe — away.
Using the cluster catalog and WMAP’s three-year view of the microwave background, the astronomers detected bulk cluster motions of nearly 2 million miles per hour. The clusters are heading toward a 20-degree patch of sky between the constellations of Centaurus and Vela.