HOW TO GET LIGHT FROM THE DARK AGES

Transcription

1 HOW TO GET LIGHT FROM THE DARK AGES Anthony Smith Lunar Seminar Presentation 2/2/2010

2 OUTLINE Basics of Radio Astronomy Why go to the moon? What should we find there?

3 BASICS OF RADIO ASTRONOMY Blackbody Radiation: B 2h c ( 2 3 ) h kt e 1 1 Rayleigh-Jeans Approximation: h e h kt B 1 kt 1 2kT 2 h kt 1 h kt Thus the blackbody function becomes:

4 For a total power receiver: ΔT T A Predetection Section T RT G ν Square- Law detector -V D The amplifier power output: W G ( T T) sys Low-pass Amplifierintegrator G LF ν LF Recorder The detector has an output voltage that varies with input voltage squared. This means that the detector d-c output voltage is directly proportional to the input power or: V D V G kt sys G k T

5 Because ΔV << V D i.e. ΔT << T sys high amplification of ΔV is needed to get a reading on the recorder. To make this easier an external voltage of V D is introduced leaving ΔV as the signal output voltage. Thus the signal power becomes: W ' C ' ( k T ) A detector output voltage also exists generating a power density of: W LF G LF 2C ( kt sys The corresponding noise from ΔT is: ' ) 2 2 LF W ' G LF C ' ( k T ) 2

6 ΔT min =T rms the minimum detectable signal is defined to be the ΔT which produces a power equal to the noise output power W LF. Thus setting the equations equal: T min Given that 0 0 G G T sys ( ( ) d ) 2 2 d 2 LF And the gain function of an ideal integrator: G( ) 2 sin (0.5 (0.5 t LF LF t ) LF 2 0 ) G G LF ( LF ) d (0)

7 Thus Δν LF =0.5/t LF Combining this with the T min and altering the equation slightly for a dipole array yields: T rms KT t LF N(N And then substituting in the Rayleigh-Jeans Approximation: B min 2 2kKT sys sys tn( N 1) 1)

8 Earlier we assumed that the T rms was much less than the system temperature. In the Very Long Wavelength VLW this is true because the system temperature is dominated not by the mechanics but by the sky temperature.

9 WHY GO TO THE MOON?

10 WHAT S WRONG WITH THE EARTH? The same effect creating the high sky temperatures also occurs in Earth s ionosphere to a much greater degree. Beyond 30MHz, the plasma frequency of the ionosphere kicks in and the atmosphere becomes opaque

11 Worse than that, fluctuations in the electron density lead to high magnitude scintillations all the way up to the 300MHz range. The moon does have an ionosphere of electrons but its much less dense than that of the Earth producing a lower plasma frequency and thus a lower cutoff. n e

12 RADIO FREQUENCY INTERFERENCE (RFI) One of the biggest problems in radio astronomy is telling the difference in local and astronomical sources.

13 The lunar far side always faces away from the Earth creating a dead zone with much less interference. RAE

14 GALACTIC SYNCHROTRON EMISSION A lunar array may eliminate RFI and lower the plasma frequency but it doesn t eliminate synchrotron emission which is 10 6 times brighter than the modeled 21-cm line. Despite this hurdle, it should be noted that astronomers working on the CMB get a signal out of the same amount of noise.

15 COSMOLOGY After the big bang at ~z=1000 the universe cooled and entered the cosmic dark ages.

16 Finding the 21 cm line both at the epoch of reionization and during the dark ages will help constrain cosmological models. Epoch of Reionization: Gas heated above CMB by X-rays Early Dark Ages: Collisions cool gas faster than the CMB Late Dark Ages: Population III stars emit Lyα and recouple spin temperature to gas

17 Given that the signal is expected to be redshifted to less than 100MHz it would be nearly impossible to find on the Earth but a single lunar dipole could find the Cosmic Dark Ages signal in 40 days.

18 DARK MATTER DECAY DETECTION Another wonderful thing about the dark ages is the simplicity of the universe. This allows a unique chance to look for exotic heat signatures since without stars to provide heat even very small sources could be discovered One possible source that astronomers are on the lookout for is dark matter decay. As the dark matter decays it should heat the gas and alter the profile of the 21-cm line in a measureable way.

19 EXTRASOLAR PLANET DETECTION Planets with large magnetic fields bend the particles of the local solar wind generating electron cyclotron masers Information from the magnetic field s signature can constrain information on the planet s composition, rotation period, number of satellites, and habitability.

20 OTHER AREAS OF RESEARCH Surveying for Radio galaxies and quasars that are very bright at long wavelengths Discovering high redshift sources Pinning down the age of quasars and galaxies with jets Constraining galaxy cluster formation models Identifying the structure of the ISM Analyze the properties of the Warm Interstellar Medium Mapping electron densities to find the source of cosmic rays Ultra high energy cosmic rays Detection of hadronic cosmic rays Detection of neutrinos Detection of meteoritic impacts

21

22 NECESSARY STEPPING STONES I. Start with a pathfinder mission with a single antenna. This should be able to measure the effects of the moon s ionosphere, the electrical properties of the surface, confirm the RFI environment and detect the redshift of the EoR. II. III. IV. Send a two element interferometer to prove lunar interferometry and do a simple sky survey. Send a three element telescope to look for high energy particles. Build a element telescope over km to look for ISM tomography, planetary bursts, and to do extragalactic surveys. V. Construct the full element telescope over tens of kilometers to finally investigate the cosmic dark ages and measure the magnetic bursts of extrasolar planets.

23 OPEN QUESTIONS How does the moon s ionosphere vary? What are the electrical properties of the lunar surface? What effects do meteoritic impacts have on measurements? How does the optical depth of the ISM vary? How much scattering do the IPM and ISM do at frequencies?

24 Questions?

25 REFERENCES Jester S. and Falke, H. (2009) Science with a lunar low-frequency array: From the dark ages of the Universe to nearby exoplanets. New Astronomy Reviews, 53, Pritchard, J. and Loeb, A. (2008) Evolution of the 21 cm signal throughout cosmic history. Physical Review D, 78, Zarka, P. (2000) Plasma interactions of exoplanets with their parent star and associated radio emissions. Planetary Space Sciences, 55, Kraus, John D. (1986). Radio Astronomy. Powell, Ohio: Cygnus-Quasar Books