PaST: Primeval Structure Telescope


Project Overview

Purpose and Use

PaST will generate a detailed, high-resolution image of a five-degree portion of the sky for radio frequencies from fifty to two-hundred Mega-Hertz.  This image will contain a timeline that details how ionized the universe was during its early stages of development: from about 200 million years to one-billion years after the Big Bang.  This means that PaST will allow cosmologists to see as much as five times closer to the start of the Big Bang than is possible with current optical or radio telescopes so that we can witness such events as the birth of the first stars in the universe.  To accomplish all this, we will "focus" PaST on either the North or South Pole.  To achieve the desired sensitivity, the exposure time will be on the order of days or weeks.

Physical Description

The PaST array will consist of about ten-thousand, log-periodic antennae (please refer to the paper "The PrimevAl Structure Telescope" by Peterson et alii for more details).  These antennae are sensitive to a wide range of frequencies, are inexpensive, are easy to construct, and offer a substantial collecting area.  Each log-periodic antenna has an effective collecting area of fifteen square-meters.  Since the common television antenna is log-periodic, we will essential use ten-thousand television antennae to collect our cosmic signals.  These antennae will be spread over several square-kilometers and be clustered into about ten to forty pods.  Within each pod, summing circuits will combine the signals to form one signal for each pod that behaves as if it were from one extremely large, very sensitive antenna.  Furthermore, the signal is amplified by a series of very low-noise, electronic amplifiers; we use refined versions of the electronics found in televisions and cellular telephones.  The signals are then passed via optical fiber to a correlator which, at high speeds, reads the information from each pod and converts it into a digital signal.  All these signals are then fed to a central computer, an array of about one-hundred personal computers, which combines and then stores information from all of the pods.  In the end, all of this information is averaged so that random, background noise is removed and the desired signal gradually emerges. 

The Science Behind the Observations
(Abridged from "The PrimevAl Structure Telescope" by Peterson et alii

The period of time that the Primeval Structure Telescope will be able to observe (roughly 200 million to one-billion years after the Big Bang) is often referred to as the "Dark Ages" since no stars were thought to exist during this period.  Recent observations, though, indicate that this name may have been dubbed in error: evidence has surfaced that suggests that the first stars formed earlier than previously thought. With PaST, we will examine this exciting period of cosmic history and define the era of the first stars.  Note that we use the term "stars" here to represent any strong source of Ultraviolet radiation, even though these objects may bear little resemblance to today's stars.

The Wilkinson Microwave Anisotropy Probe (WMAP) has provided a wealth of information on the state of the early universe, but the most intriguing and unexpected result from this satellite was the hint that the Universe may have been ionized very early: perhaps as early as 200 million years after the Big Bang.  The WMAP data does not give any indication as to what the source of that energy might be, but if the energy source were gravitational or nuclear, the ionizing radiation would have been produced in compact, star-like objects.  The vestiges of such early ancestors to modern stars could be evident in a "patchy" image of the ionization of the sky.  PAST will map early ionization by detecting the presence or absence of neutral hydrogen emission lines since the glow of hydrogen (or any element) has uniquely identifiable frequencies. 

It is already know that the very early universe was ionized by the Cosmic Microwave Background (CMB), and we also know that the universe is ionized today.  The ionization of the modern universe primary results from such objects as stars and quasars.  Before the WMAP ionization result, many cosmologist accepted the simplest ionization history that fit the data.  This model of history has three eras:
  1. An ionized universe as a result of the rather hot Cosmic Microwave Background.
  2. A long period of very little ionization.
  3. The advent of stars and quasars which are gradually reionizing the universe. 
However, if the WMAP result is right, this story is incorrect or incomplete.  One possibility is that the first stars formed very early on and the universe has been partially ionized since.  Others have suggested that early stars formed and died rather quickly, but still resulted in a period of ionization in the early universe. 

In either scenario, or even if the WMAP result is in error, PaST will be able to detect the transitions between neutral and ionized states.  There should be about a twenty milli-Kelvin cosmic glow attributable to warm, neutral hydrogen in the Very High Frequency (VHF -- thirty to three-hundred Mega-Hertz -- home to channels two through thirteen in broadcast television) range.  A neutral hydrogen atom is a single proton orbited by a single electron.  Both of these particles can be thought to be rotating and, since they both have charge, producing a magnetic field.  The most stable state is for the two particles to be spinning in the same direction (clockwise or counterclockwise).  Occasionally, the hydrogen gains some energy from the environment in the form of a collision with another particle or the absorbtion of a photon.  In such cases, the atom gains energy, some of which allowing the electron and proton to spin in opposite directions.  This is an unstable state that will quickly decay back to the aligned spin state.  The energy that kept the hydrogen in the unaligned state, though, must go somewhere, so it is released into form of a photon.  Since this transition is the same for all hydrogen atoms (they are all identical), the photon will have the same frequency and wavelength for all hydrogen atoms: a 1420 Mega-Hertz frequency and a twenty-one centimeter wavelength. 

In the young universe, as clouds of neutral gas began to gravitationally collapse, temperatures rose to the range of one-thousand to ten-thousand Kelvin.  These temperatures were sufficient to induce the aforementioned spin flips in neutral hydrogen.  The gravitational collapse would produce hot clumps of matter with anywhere from ten-thousand to one-million times the mass of the sun.  PaST will not be able to locate individual clumps: rather, it will see a diffuse glow that spans the sky when viewed in the VHF range. 

When hydrogen ionizes, though, the proton and electron separate and their spins are no longer coupled.  As stars begin to light up and ionize their portions of the sky, the signals corresponding to the spin transitions will gradually disappear so that the background glow will fade into patchiness and then into relative darkness.  This transition has been predicted in various cosmological simulations, including the one that produced the image below.


The nine panels show simulated sky brightness in redshifted twenty-one centimeter emission, separated in time, beginning in the upper left. Already in the first time slice the densest region, at the right of the panel, is losing brightness as the first stars are beginning to ionize the hydrogen gas. By the final panel the entire sky is dim. Note the high contrast in the intermediate images. Furthermore, note that the lightest patches (top center of the first panel for example) are the first to become dark: they glow brightly since they are hot but quickly become dark as fussion begins.  Plot provided by S. Furlanetto.

Of course, since the universe is expanding, the emissions of these spin flips will be red-shifted meaning that their frequencies will decrease and their wavelengths increase.  The further we look back in time, the more dramatic the red-shift.  Therefore, by designing PAST to make observations in a particular range of frequencies, we can study a particular era in the history of the universe. 

One problem is that warm, neutral hydrogen in not the only source of VHF emissions.  The Galaxy itself has a brightness of one-thousand Kelvin at forty Mega-Hertz which declines to about forty Kelvin at 210 Mega-Hertz.  Behind this "blaze," we will attempt to detect patches with a temperature contrast of about twenty milli-Kelvin and with a typical size of five arc-minutes.  Fortunately, the low frequency spectrum of Galactic emissions is featureless and smooth: the galaxy has very little structure at the five arc-minutes angular scale. Therefore, the galactic emissions can easily be averaged out, leaving behind the red-shifted signals of neutral hydrogen. 

Similar extraction procedures can be used to remove radio signals from sources located outside our galaxy.  Even if all of the signals cannot be removed in this fashion, the sharpness of the patches is great enough to be clearly evident even with the other noise. 

PAST has been designed taking all these and more factors into account.  The use of log-periodic antennae allows for a large effective sampling area and a wide range of frequencies.  Location of the telescope is critical since it must be as far away from terrestrial VHF sources as possible: including television and radio transmitters.  If a site is too noisy, the underlying signal will begin to be distorted and lost by the overpowering terrestrial signal: a signal that is well-recieved on a television is millions or billions of times stronger than this background radiation. 

Key Specifications of the Planned PaST Instrument

Total Antennae
Effective Elements (Number of Pods)
10 - 20
Effective Area
70,000 Square-Meters
Sky Brightness Temperature
90 Kelvin
Instantaneous Imaging Field View
2 Square-Degrees
Angular Resolution
3 Arc-Minutes
Instantaneous Frequency Coverage
50 - 200 Mega-Hertz
Frequency Resolution
4 Kilo-Hertz