Huge Lenses to observe Dark Energy

Dark energy is one of the stuffs in the universe we know what we don’t know. Out there in the universe, we know there is gravity trying to pull things together, but there also exists another mysterious force – we called it “dark energy” – that is trying to pull things apart, which we’re not really sure what it is.

Dark energy entered the scene in 1998. That year, two teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search, who were making a survey of supernovae in very distant galaxies in order to measure the expansion rate of the universe with time, found that instead of slowing down, the universe expansion is accelerating!

Supernova in NGC4526

Supernova 1994D in Galaxy NGC4526. Credit: NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team

We can use supernovae, specifically type Ia supernovae, to measure distances to galaxies. Type Ia supernova happens when a white dwarf in a binary system stealing mass from its normal companion star reaches the Chandrasekhar limit (critical mass) and explode.

Since all white dwarfs achieve the same critical mass before exploding, their luminosities are almost the same and hence can be used as “standard candles” to determine their distances. If they appeared fainter, then it means that the supernova is further away. If we know the distance to a supernova (say 1 billion light-year away), then we can know how long ago it occurred (1 billion years ago).

To measure the expansion rate of the universe with time, we required another piece of information: the redshift of the supernovae. As the universe expands, the light from the supernova is stretched or redshifted (its wavelengths become longer). By looking at how much the light is stretched, we can know how fast the supernova is moving away from us.

redshift

In 1916 Vesto Slipher observed about 50 nearby galaxies, spreading their light out using a prism, and recording the results onto film. The results confounded him and the other astronomers of the day. Almost every object he observed had its light stretched to redder colors, indicating essentially everything in the universe was moving away from us. Here we show the spectrum of a galaxy as Slipher would have seen it. The light is stretched in the bottom spectrum, so that the dark lines (the colours where elements such as sodium absorb light), are stretched to redder colors. Image and caption by Brian Schmidt (leader of the High-Z Supernova Search Team).

Now, by combining the distance and the redshift of a supernova, we can know at a point in time (given by the distance) how fast a supernova is moving away from us (given by the redshift).

If the universe expansion is slowing down, a supernova 1 billion years ago should be moving slower than a supernova 2 billion years ago and a supernova 2 billion years ago should be moving slower than a supernova 3 billion years ago. The expansion is getting slower and slower over time. The opposite goes if the universe is accelerating. Hence, by studying many supernovae at different distances, we can know the rate of expansion with time.

Another way to look at the expansion rate is that if the supernovae are brighter than their redshift indicate, it means that they are nearer than they should have been, hence they are moving away slower than expected, so the expansion of the universe is slowing down.

What the two teams of astronomers found completely changed the way we look at the universe. We long know that the universe is expanding since Hubble’s discovery in 1929, but we are expecting the universe expansion to gradually slow down due to gravity. Instead, the astronomers found that the supernovae are fainter than expected, meaning they are farther away than they should have been and further means that the expansion of the universe was accelerating instead of slowing down as what we expected!

This mysterious force that is causing the universe to fly apart faster and faster is something everyone now calls dark energy. No one understands what it is…

At first, the result was questioned by other researchers: is it possible that the supernovae were dimmed by obscuring interstellar dust lying between us and them? Or maybe the supernovae themselves were intrinsically dimmer and different in the past. But with more data and careful checking, those explanations were discarded; the universe is accelerating, and the dark energy hypothesis has held up.

Another piece of evidence comes from the observation of the Cosmic Microwave Background (CMB) which also strongly suggests that the universe is speeding up.

This stunning discovery told us that there is a whole lot of the universe we don’t know what it is. The result suggests that 4% of the universe is made of ordinary matter – meaning you and me and planets and stars and galaxies and everything we know. The other 22% is dark matter – matter that is weakly interacting and invisible but its presence can be felt by its gravitational effects on surrounding visible matter; something that we’re still trying to figure out what is it. And lastly we are left with a big portion of what we don’t know – 74% is dark energy.

Astronomers are not happy. How can we be comfortable that we only know 4% of our universe?

And we determine to seek for answers…

Enter the Dark Energy Survey

This is a next generation sky survey aimed directly at understanding the mystery of dark energy. An international group of astronomers are constructing a huge sophisticated camera to look for it, with the largest one at one metre in diameter, making it one of the largest in the world.

Recently they have reached a milestone in the construction of the camera. The pieces of glass for the five unique lenses of the camera have been shipped from the US to France to be shaped and polished into their final form. Each milestone will brings us closer to the understanding of dark energy.

The lenses will be polished to a smoothness level of one millionth of a centimetre. Then they will be sent to the Optical Science Laboratory at UCL in London for assembly into the camera and from there to the telescope in Chile, where observations will start in 2011 and will continue until 2016.

Click to enlarge
The largest of the five lenses

The Dark Energy Survey (DES) camera will map 300 million galaxies using the Blanco 4-meter telescope – a large telescope with new advanced optics at Chile’s Cerro Tololo Inter-American Observatory. The vast DES galaxy map will enable the astronomers to measure the Dark Energy far more precisely than current observations.

Prof. Ofer Lahav, head of the UCL Astrophysics Group, who also leads the UK DES Consortium, commented “Dark Energy is one of the biggest puzzles in the whole of Physics, going back to a concept proposed by Einstein 90 years ago. The DES observations will tell us if Einstein was right or if we need a major shift in our understanding of the universe.”

Extra: Hubble Dark Energy Site

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~ by thChieh on June 29, 2008.

One Response to “Huge Lenses to observe Dark Energy”

  1. […] Huge Lenses to observe Dark Energy « My Dark Sky […]

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