## Three space telescopesMay 22, 2008

Posted by dorigo in astronomy, cosmology, news, physics, science.
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This morning at PPC2008 the audience heard three different talks on proposed space missions to measure dark energy through supernova surveys at high redshift and weak lensing observations. I am going to give some highlights of the presentations below.

The first presentation was by Daniel Holz on “The SuperNova Acceleration Probe“, SNAP.

SNAP is all about dark energy (DE). Supernovae show there is acceleration in the universe. However, to measure precisely the amount of DE in the universe one is required to determine the distance versus redshift of supernovae of type 1A. These are collapsing carbon-oxygen white dwarfs at the Chandrasekhar limit, i.e. with exactly the right amount of mass: they make a very well understood explosion when they die, and they can thus be used as standard candles to measure distance and their redshift tells us how much they are receding from us. The precision required to extract information on DE is at the level of a few % or so, which is very difficult to do. So one needs a great control of systematics. One also wants to distinguish DE from modified gravity models, which accommodate some of the observed features of the universe by hypothesizing that the strength of gravity is not exactly the same as one goes to very large distance scales.

Separating out the different models is not easy. Supernovae allow to determine the integrated expansion – how much the universe accelerated since its origin; the origin of growth of structure in the universe is not measured there. The way snap is approaching this is by combining SN measurements with weak lensing. Weak lensing is the deviation of photons from a distant source when passing through large amounts of mass, like a cluster of nearby galaxies.

SNAP is a space telescope (see picture). Its strong point is the width of the field of view it can imagine at a time: 0.7 squared degrees of sky, much larger than that of the Hubble space telescope. Also, it provides for lots of color information: it goes into the infrared, and it has 9 filters to get spectral information from the observed objects.

SNAP aims at obtaining 2000 supernovae of type 1A at redshift z<1.7, and to do a weak lensing survey over 4000 square degrees. It is a 1.8m telescope, diffraction limited. The bottom line for SNAP is to measure $\Omega_{DE}$ to 0.4%, and the parameters $w_0$ to 1.6% and $w'$ to 9%.

After Holz, R. Kirshner described “Destiny, the Dark Energy Space Telescope“. Destiny is a similar concept to the previous one. Its science goal is to determine the expansion history of the universe to 1% accuracy.
It is a 1.65m wide telescope – slightly smaller than SNAP (1.8m). The program has to cost 600 million dollars or less including cost. It is receiving a green light from its funding agencies, NASA and DOE. It will operate from the same location as SNAP -a lagrangian orbit point called “L2”, which is not affected much by gravitational effects from the earth and the moon. Kirshner made me laugh out loud as he added that despite the location is the same as that of SNAP, it will not be too crowded a place, since SNAP won’t be there.

Kirshner explained that the project is very conservative. They do not need low redshift SN measurement from space. From space one can work in near IR, something that can only be done there. It complements well with ground-based telescopes. A very distinctive difference with SNAP is that Destiny is an imaging spectrograph. It takes a spectrum of every object in the field every time.
For SNAP you need to make a choice to what object to take a spectrum. The resolution in wavelength is $\lambda/(\Delta \lambda=75$, equivalent as having 75 filters of spectral energy distribution.

The Destiny philosophy is to keep it simple, stupid. It is a satellite for which every piece has flown in space previously: nothing we do not know we can do already. It uses the minimal instrument required to do the job. And it does in space what must be done in space, without taking the job away from ground based observations. Also, a point which was emphasized is that there will be no time-critical operations: it is a highly automated, fixed program. No need for 24/7 crews on earth to decide what star to take spectra of.

DE measurements require to measure both the acceleration and deceleration epochs, z<>0.8. On the
ground magnitudes accessible for SN are lower than 24, on space they are higher, at z>0.8.
To take a picture of the time derivative of the equation of state, you need to measure over the
jerk region in the distance-to-redshift diagram, where the curve changes from acceleration to deceleration, at z about 1. This you do from space.

Kirshner explained that the sky is dark at night in the optical wavelengths, but at IR it is ten to a hundred times brighter. In space you go down by a factor of 100 in brightness. But there is also absorption in the atmosphere, mostly water vapor. From the ground you can work at small intervals of wavelength: these for the infrared lie at 1.4 micrometers and 1.85 micrometers. In space you can look at the entire range.