The obvious questions and their brief answers, for those wanting to know what’s going on today. If you already know roughly what’s going on and want the bottom line, read the answer to the last question.
You may want to start by reading my History of the Universe articles, or at least having them available for reference.
The expectation is that today we’re going to hear from the BICEP2 experiment.
- What is BICEP2?
BICEP2, located at the South Pole, is an experiment that looks out into the sky to study the polarization of the electromagnetic waves that are the echo of the Hot Big Bang; these waves are called the “cosmic microwave background”.
- What are electromagnetic waves?
Electromagnetic waves are waves in the electric and magnetic fields that are present everywhere in space. Visible light is an electromagnetic wave, as are X-rays, radio waves, and microwaves; the only difference between these types of electromagnetic waves is how fast they wiggle and how long the distance is from one wave crest to the next.
- What is the cosmic microwave background [CMB for short]?
The glow leftover from the Hot Big Bang. The part of the universe we can observe today (the “observable patch”; the universe as a whole may be much larger) was once very hot, during the Hot Big Bang. When it was hotter than about a few thousand degrees, it was opaque to light. But once it cooled sufficiently for atoms to form, it became transparent, and any light, and any other forms of electromagnetic waves, that was still being emitted at that time was then free to stream across the universe forever. The “glow” from that hot period thus is still present in the universe. As the observable patch expanded, the wavelength of the electromagnetic waves increased; today most of those waves are in the range of microwaves.
- What is polarization?
An electromagnetic wave has the property that as it heads from point A to point B, the electric field always points in a direction perpendicular to its motion. (The magnetic field points perpendicular both to the direction of motion and to the electric field.) The direction along which the electric field points is the “polarization” of the electromagnetic wave.
- What are photons?
Electromagnetic waves (including visible light) are made from particles (or “quanta”) called “photons”. Each photon is an electromagnetic wave of minimal possible intensity, and each photon’s polarization can be individually measured.
- What precisely is BICEP2 doing?
BICEP2 looks in each direction of the sky, and detects cosmic microwave photons from that direction. It then determines whether the polarization of the photons from that direction is entirely random, and thus has an average of zero, or whether it has a (very tiny!) tendency for a preferential orientation.
If the photons coming from a single small region of the sky have a random polarization direction, then the cosmic microwave background from that region is “unpolarized”. But if they have a slightly preferred direction for their polarization, then the cosmic microwave background from that region of the sky is “polarized”, and the amount and orientation of the preference is the “polarization” that BICEP2 is measuring.
- Why is BICEP2 making this measurement?
Both the average energy (“temperature”) of the photons coming from a particular direction and their average polarization tell us something about what the observable patch of the universe was like a long time ago: 380,000 years after the Hot Big Bang, which is about when the observable patch became transparent.
- Why do we care about that?
Because what the universe was like 380,000 years after the Hot Big Bang can in turn be used to learn about the very beginning of the Hot Big Bang, and what preceded it — which may have been a period of cosmic inflation.
- What does the temperature of the photons from the cosmic microwave background [CMB] tell us?
The temperature of the CMB photons is, amazingly, almost completely uniform across the sky. Once the motion of the Milky Way is accounted for, the temperature varies only at one part in 100,000. The pattern of temperature variation across the sky — first measured by the COBE satellite, and with increasingly precision since then, most recently by the Planck satellite — tells us how non-uniform the universe was 380,000 years after the Hot Big Bang began. From these non-uniformities, working both forward in time through the era of galaxies that we live in, and backward in time through the early Big Bang, scientists have been able to infer many properties of the universe with unprecedented precision, determining how long it has been since the Hot Big Bang, and the make-up of the observable patch (i.e. how much dark energy, dark matter and ordinary matter it contains), to a spectacular degree. These measurements could have ruled out the possibility of cosmic inflation, but instead they are (so far) quite consistent with that possibility.
- What does the period at and just before the Hot Big Bang have to do with small non-uniformities in an otherwise uniform temperature 380,000 years later?
Whatever non-uniformities were present when the Hot Big Bang started would have persisted and changed in a way that scientists can calculate, leading to non-uniformities of a predictable size at 380,000 years. In other words, from knowing what the non-uniformities are at 380,000 years, scientists can work backwards, and determine what the non-uniformities were when the Hot Big Bang began.
- Why would there have been non-uniformities in the temperature of the Hot Big Bang?
Why not? In fact, the first question you should ask is why they’re so small!
- Why are they so small?
Well, we’re not sure, but that’s where the idea of inflation comes in. By causing the observable patch of the universe and its surrounding regions to blow up almost instantly into a vastly larger size, inflation would have pushed all material and all structure far, far away, making the observable patch incredibly uniform.
- But then why are there any measurable non-uniformities at all
Ah — because of how quantum mechanics combines with inflation. (This isn’t simple, and it deserves an article in future.) Inflation is caused by the presence of a substantial amount of dark energy. That dark energy is associated with a field (or fields), called the inflaton (or inflatons), about which we currently know almost nothing. But we do know this:In quantum mechanics, no field or object is ever truly constant or stationary. There is always a sort of quantum jitter which causes it to be a bit uncertain. The inflaton field’s value undergoes this quantum jitter. As a consequence, the dark energy that is present during inflation is not exactly constant throughout space; and that, finally, means that there are small non-uniformities in the expansion of space, which in turn lead to small non-uniformities in the Hot Big Bang, which eventually become visible as non-uniformities in the temperature of the CMB photons.
- What’s the point of looking at CMB polarization?
The pattern of non-uniformities in the polarization can tell us some things about the universe that are different from what the non-uniformities in the temperature have already taught us.
- Why is there any polarization at all? Why aren’t the CMB photons completely random?
Just as the universe is becoming transparent about 380,000 years after the Hot Big Bang, and electrons are all being captured by atomic nuclei as atoms form, the last photons to hit something often scatter off electrons, a process called “Thomson scattering”. Thomson scattering has the property that if a bunch of unpolarized photons come in and hit an electron, but there are more of them coming from above and below than from the left and from the right, then the photons which emerge forward will be somewhat polarized. In short, Thomson scattering can convert a non-uniformity in the unpolarized photons that are present as the universe becomes transparent to a polarization effect that experiments like BICEP2 can observe.
- Since the non-uniformities in the CMB photons are so small, doesn’t that mean that polarization effect that BICEP2 has to measure will be extremely tiny?
You bet! BICEP2 is making a very difficult measurement. An unprecedented one! But of course, that’s why they might make a discovery!
- What will the polarization tell us that the temperature non-uniformities haven’t already told us?
Well, there are two types of polarization in the polarization pattern: “E-mode” and “B-mode”. E-mode was measured some time ago, and doesn’t tell us that much new (though it would have if its features had been surprising; they weren’t.) B-mode, however, can tell us a lot.
- What’s all this about E-mode and B-mode polarization?
The polarization has to be separated into two classes of patterns. If you look in one direction in the sky, and you look at the polarization of the CMB photons nearby to that direction, you may find that the pattern has a form that would look the same if you reflected it in a mirror. That type of polarization pattern is called “E-mode”.
Or you may find that the pattern is of a form that would flip over in a mirror; that type of polarization pattern is called “B-mode”.
Typically you’ll find a mix of the two; the pattern is almost but not quite unchanged in a mirror.
But (as previous experiments have already shown) the amount of B-mode is much less than the amount of E-mode, making BICEP2′s measurement of B-mode even more difficult!
- Why is B-mode polarization able to tell us something that neither E-mode nor temperature non-uniformities can tell us?
E-mode polarization is sensitive to the same type of effects that cause temperature non-uniformities. But B-mode comes from two sources.
1) In small patches of sky, B-mode polarization arises from a combination of E-mode polarization present 380,000 years ago and the gravitational lensing (i.e., bending of light) of the CMB photons by galaxies that the CMB photons have passed near on their journey to Earth. B-mode polarization on these smaller scales, which was detected about a year ago, thus tells us something about properties of the observable patch after 380,000 years post-Hot-Big-Bang.
2) But across larger swathes of the sky, B-mode polarization arises in a novel way. Non-uniformities in the energies of the photons as the universe was becoming transparent also were potentially due to the presence of gravitational waves — ripples in space itself. Unlike non-uniformities merely due to there being slightly more dense and less dense regions in the Hot Big Bang, which only give rise to mirror-symmetric (E-mode) polarization after Thomson scattering, gravitational waves can give rise both to mirror-symmetric and mirror-asymmetric polarization… to both E-mode and B-mode. Thus, B-mode polarization on large scales tells us about non-uniformities due to gravitational waves that may have been present 380,000 years after the Hot Big Bang — and a discovery of non-zero B-mode polarization on large scales would be a measurement of powerful gravitational waves present in the early universe!
- Wow! Ripples in space and time! Gravitational waves predicted by Einstein’s theory of gravity! Is this the first detection of gravitational waves?
No. This detection is indirect; we actually measure polarization of light and only infer gravitational waves are present. The 1993 Nobel Prize was for the indirect detection of gravitational waves in careful measurements of a system of two neutron stars (the Hulse-Taylor pulsar system). Efforts toward direct detection of these waves are underway and may bear fruit in the near future, but not yet.
- What’s so important about these gravitational waves?
Their existence is predicted by inflation; just as the inflaton has quantum jitter, so do space and time themselves, and this leads to ripples in space and time. And so their presence, if in the appropriate pattern, would be more strong evidence in favor of inflation of certain types, and evidence both against certain non-inflationary ideas and against certain variants of inflation that don’t predict large amounts of gravitational waves.
Moreover, their properties — in particular, their total power — would measure, for the first time, the amount of dark energy that was driving inflation, and therefore how rapidly inflation was occurring. And also, because this dark energy is what eventually heats the universe and starts the Hot Big Bang, it would tell us how hot the universe became after inflation and before it began to cool.
So this is a very big deal!