What is intensity mapping?
Most telescopes try to measure the properties of individual luminous sources, observing a star, galaxy, or planet, for example. But this presents a substantial challenge for the Cosmic Dawn, because those galaxies are so extraordinarily distant and faint. Even the James Webb Space Telescope will see only a fraction of the total star formation in the Universe at these early times, because most of it occurs in tiny sources too faint to be observed individually. A partial solution to this problem is intensity mapping, in which a telescope observes the sky with such blurry vision that it cannot distinguish individual galaxies. At first, this seems counterproductive, but imagine a panel of light bulbs placed very far from you. Although you cannot make out the individual lights, the panel does not disappear - the light still streams toward you, it is simply that you cannot make out the individual bulbs. Intensity mapping uses the same idea to study the Cosmic Dawn: even though the telescope cannot make out individual galaxies in a particular patch of the sky, it can measure all the light coming from them - including the faint ones invisible to larger telescopes!
Image: A simulated field with galaxy positions (Left) and the corresponding (CO) intensity map.
Credit: Patrick Breysse from Kovetz et al 2017
How can intensity mapping tell us about the Cosmic Dawn?
Intensity mapping provides a powerful method to observe the very faint galaxies that dominate the Universe during the Cosmic Dawn. The challenge comes in interpreting this light: how can we tell it comes from galaxies in the Cosmic Dawn rather than more nearby (but still faint) sources? The easiest way is to track special features of the light from these galaxies called emission lines, through which the galaxies can be located. The SPHEREx satellite, a NASA mission that will launch in 2024, aims to do just this using ultraviolet light from these early galaxies. It will provide a census of the light from these faint sources, and the emission line signatures will even help us understand the conditions inside these otherwise invisible galaxies.
Image: SPHEREx concept
Credit: NASA JPL
How can we use gravitational waves to study the Cosmic Dawn?
The 21st century has seen the birth of a new form of astronomy: the LIGO experiment detected the first gravitational waves from nearby black holes in 2015. Gravitational waves are ripples in space-time generated, in this case, by the collisions of black holes. While only a few gravitational wave events have been observed so far - all originating from relatively close to our Galaxy - the Universe is full of black holes, many of them millions or billions of times more massive than our Sun. If such black holes merge, they will also create gravitational waves that reach Earth. Current experiments cannot observe those kinds of events, but a future planned mission, the Laser Interferometric Gravitational Observatory (or LISA), due to launch in the 2030s, can. One of its goals is to trace the history of these black holes all the way back to the Cosmic Dawn.
Image: The LISA mission will consist of three spacecraft, using lasers to precisely measure how their separation changes.
Credit: © NASA/Simon Barke
What can cosmic microwave background telescopes tell us about the Cosmic Dawn?
The cosmic microwave background is our primary tool for understanding the earliest phases of the Universe’s history, as described here. Although these microwaves were generated long before the Cosmic Dawn (just 400,000 years after the Big Bang!), to reach us they must pass through that era. Microwave photons are unaffected by intergalactic hydrogen while it remains neutral, but once the reionization process begins, the microwaves can bounce - or scatter - off of electrons liberated by reionization. Because this bounce changes the microwave’s direction of travel, it blurs out a telescope’s images of the cosmic microwave background - but it can also leave signatures against that same background! The scattering process generates a property called polarization in the cosmic microwave background, which can be detected by specialized telescopes. (Polarization means that the direction in which the light waves oscillate is aligned - it also happens when light bounces off a surface, like water. This is why polarized sunglasses help lifeguards keep watch - they can see the surface without as much glare!) The Planck telescope has measured the amount of polarization, which has helped astronomers determine that reionization occurred about a billion years after the Big Bang. The reionization process also has other effects on the cosmic microwave background, distorting it as the ionized bubbles grow and merge. Astronomers are measuring this aspect as well, and through it they hope to determine the duration of the reionization era.
Image: Concept of the Planck Spacecraft
Credit: ESA/NASA/JPL-Caltech
Which telescopes can study other aspects of galaxies during the Cosmic Dawn?
Many other telescopes can (or will) study the Cosmic Dawn as well, including both other radio telescopes and future X-ray telescopes. One of the key unknowns about early galaxies is how they form stars from their swirling gas clouds. Radio observatories, like the Atacama Large Millimeter Array, or ALMA, can help diagnose these processes by looking for light produced by molecules that form in dense, star-forming clouds and by dust produced as the galaxies evolve. ALMA has already studied many of these galaxies, finding tantalizing clues about their complex internal environments. Existing X-ray telescopes are not sensitive enough to study the Cosmic Dawn in detail, but a future space telescope, ATHENA, hopes to measure the X-rays produced as black holes swallow nearby gas in these distant galaxies.
Image: ALMA Observatory
Credit: © EFE/Ariel Marinkovic
