Out of the Darkness
JWST will be able to see further back than ever before, to when the first bright objects (stars and galaxies) were forming in the early universe. Credit: STSci
Why is a powerful infrared observatory key to seeing the first stars and galaxies that formed in the universe? Why do we even want to see the first stars and galaxies that formed? One reason is... we haven't yet! The microwave COBE and WMAP satellites saw the heat signature left by the Big Bang about 380,000 years after it occurred. But at that point there were no stars and galaxies. In fact the universe was a pretty dark place.
The Early Universe
After the Big Bang, the universe was like a hot soup of particles (i.e. protons, neutrons, and electrons). When the universe started cooling, the protons and neutrons began combing into ionized atoms of hydrogen (and eventually some helium). These ionized atoms of hydrogen and helium attracted electrons turning them into neutral atoms - which allowed light to travel freely for the first time, since this light was no longer scattering off free electrons. The universe was no longer dark! But we still don't really know what the universe's first light, created by sources (stars) that fused these hydrogen atoms into more helium, looked like.
Imagine light leaving the first stars and galaxies nearly 13.6 billion years ago and traveling through space and time to reach our telescopes. We're essentially seeing these objects as they were when the light first left them 13.6 billion years ago. Because the universe is expanding, the farther back we look, the faster these objects (like the first stars and galaxies) are moving further away from us, which means that their light is being shifted towards the red. Their light is what we call "redshifted."
JWST will be able to see back to about 200 million years after the Big Bang. But why do we need to see infrared light to understand the early universe? Because light from these objects is shifted to the red. Credit: Aleš Tošovský
Electromagnetic Spectrum Characteristics Credit: NASA
Redshift means that light that is emitted by these first stars and galaxies as visible or ultraviolet light, actually gets shifted to redder wavelengths by the time we see it here and now. For very high redshifts (i.e., the farthest objects from us), that visible light is generally shifted into the near- and mid-infrared part of the electromagnetic spectrum. For that reason, to see the first stars and galaxies, we need a powerful near- and mid-infrared telescope, which is exactly what JWST is!
JWST will address several key questions to help us unravel the story of the formation of structures in the Universe such as:
- When and how did reionization occur?
- What sources caused reionization?
- What are the first galaxies?
JWST's Role in Answering These Questions
To find the first galaxies, JWST will make ultra-deep near-infrared surveys of the Universe, and follow up with low-resolution spectroscopy and mid-infrared photometry (the measurement of the intensity of an astronomical object's electromagnetic radiation). To study reionization, high resolution near-infrared spectroscopy will be needed.
The Era of Recombination
Until around 400 million years after the Big Bang, the Universe was a very dark place. There were no stars, and there were no galaxies.
After the Big Bang, the universe was like a hot soup of particles (i.e. protons, neutrons, and electrons). When the universe started cooling, the protons and neutrons began combining into ionized atoms of hydrogen and deuterium. Deuterium further fused into helium-4. These ionized atoms of hydrogen and helium attracted electrons turning them into neutral atoms. Ultimately the composition of the universe at this point was 3 times more hydrogen than helium with just trace amounts of other light elements.
This process of particles pairing up is called "Recombination" and it occurred about 400,000 years after the Big Bang. Another result of this is the end of what is called the cosmic dark ages. Light had formerly been stopped from traveling freely because it would frequently scatter off the free electrons. Now that the free electrons were bound to protons, light was no longer being impeded.
The Universe went from being opaque to transparent at this point, and "the era of recombination" is the earliest point in our cosmic history to which we can look back with any form of light. This is what we see as the Cosmic Microwave Background today with satellites like the Cosmic Microwave Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). At right is an illustration of the timeline of the universe.
Illustration of the Timeline of the Universe. Credit: WMAP
The Epoch of Reionization
Another change occurred after the first stars formed. Theory predicts that the first stars were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae. The energetic ultraviolet light from these first stars was capable of splitting hydrogen atoms back into electrons and protons (or ionizing them). Observations of the spectra of distant quasars tell us that this occurred when the Universe was almost a billion years old.
That era, when the universe was a billion years old, is known as "the epoch of reionization." It refers to the point when most of the neutral hydrogen was reionized by the increasing radiation from the first massive stars. Reionization is an important phenomenon in our Universe's history as it presents one of the few means by which we can (indirectly) study these earliest stars. But scientists do not know exactly when the first stars formed and when this reionization process started to occur.
Hubble Deep Field - The first significant look back to the era of the universe when early galaxies were forming. The image is a long exposure of a very small area of the sky, which revealed a large number of very faint, and previously unseen, objects. These objects are some of the oldest and most distant galaxies and allowed us to, as Stefano Cristiani said, "glimpse the first steps of galaxy formation more than 10 billion years ago." Other deeper studies have come after, and JWST will also do deep field studies. JWST's imaging capabilties and infrared vision will show us the early universe with unprecedented clarity. Credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA
The emergence of these first stars marks the end of the "Dark Ages" in cosmic history, a period characterized by the absence of discrete sources of light. Understanding these first sources is critical, since they greatly influenced the formation of later objects such as galaxies. The first sources of light act as seeds for the later formation of larger objects.
Additionally, the first stars that exploded as supernovae might have collapsed further to form black holes. The black holes started to swallow gas and other stars to become objects known as "mini-quasars," which grew and merged to become the huge black holes now found at the centers of nearly all massive galaxies.