Many of you will have read in the last week that unfortunately (though to no one’s surprise after seeing the data from the Planck satellite in the last few months) the BICEP2 experiment’s claim of a discovery of gravitational waves from cosmic inflation has blown away in the interstellar wind. [For my previous posts on BICEP2, … Read more
I’m busy dealing with the challenges of being in a quantum superposition, but you’ve probably heard: BICEP2’s paper is now published, with some of its implicit and explicit claims watered down after external and internal review. The bottom line is as I discussed a few weeks ago when I described the criticism of the interpretation … Read more
I’m not going to drag my readers into the mud of the current discussion, both because it’s very technical and because it’s rather vague and highly speculative. Even the authors of the two papers admit they leave the situation completely unsettled. But to summarize, the main purpose and effect of these papers seems to be this:
Familiar throughout our international culture, the “Big Bang” is well-known as the theory that scientists use to describe and explain the history of the universe. But the theory is not a single conceptual unit, and there are parts that are more reliable than others.
It’s important to understand that the theory — a set of equations describing how the universe (more precisely, the observable patch of our universe, which may be a tiny fraction of the universe) changes over time, and leading to sometimes precise predictions for what should, if the theory is right, be observed by humans in the sky — actually consists of different periods, some of which are far more speculative than others. In the more speculative early periods, we must use equations in which we have limited confidence at best; moreover, data relevant to these periods, from observations of the cosmos and from particle physics experiments, is slim to none. In more recent periods, our confidence is very, very strong.
Notice that in the figure, I don’t measure time from the start of the universe. That’s because I don’t know how or when the universe started (and in particular, the notion that it started from a singularity, or worse, an exploding “cosmic egg”, is simply an over-extrapolation to the past and a misunderstanding of what the theory actually says.) Instead I measure time from the start of the Hot Big Bang in the observable patch of the universe. I also don’t even know precisely when the Hot Big Bang started, but the uncertainty on that initial time (relative to other events) is less than one second — so all the times I’ll mention, which are much longer than that, aren’t affected by this uncertainty.
I’ll now take you through the different confidence zones of the Big Bang, from the latest to the earliest, as indicated in the figure above.
Did the universe begin with a singularity? A point in space and/or a moment in time where everything in the universe was crushed together, infinitely hot and infinitely densely packed?
Doesn’t the Big Bang Theory say so?
Well, let me ask you a question. Did you begin with a singularity?
Let’s see. Some decades ago, you were smaller. And then before that, you were even smaller. At some point you could fit inside your mother’s body, and if we follow time backwards, you were even much smaller than that.
If we follow your growth curve back, it would be very natural — if we didn’t know anything about biology, cells, and human reproduction — to assume that initially you were infinitesimally small… that you were created from a single point!
But that would be wrong. The mistake is obvious — it doesn’t make sense to assume that the period of rapid growth that you went through as a tiny embryo was the simple continuation of a process that extends on and on into the past, back until you were infinitely small. Instead, there was a point where something changed… the growth began not from a point but from a single object of definite size: a fertilized egg.
The notion that the Universe started with a Big Bang, and that this Big Bang started from a singularity — a point in space and/or a moment in time where the universe was infinitely hot and dense — is not that different, really, from assuming humans begin their lives as infinitely small eggs. It’s about over-extrapolating into the past.
First things first. As with all major claims of discovery, considerable caution is advised until the BICEP2 measurement has been verified by some other experiment. Moreover, even if the measurement is correct, one should not assume that the interpretation in terms of gravitational waves and inflation is correct; this requires more study and further confirmation.
The media is assuming BICEP2’s measurement is correct, and that the interpretation in terms of inflation is correct, but leading scientists are not so quick to rush to judgment, and are thinking things through carefully. Scientists are cautious not just because they’re trained to be thoughtful and careful but also because they’ve seen many claims of discovery withdrawn or discredited; discoveries are made when humans go where no one has previously gone, with technology that no one has previously used — and surprises, mistakes, and misinterpretations happen often.
I’m still updating this post as more information comes in and as I understand more of what’s in the BICEP2 paper and data. Talking to and listening to experts, I’d describe the mood as cautiously optimistic; some people are worried about certain weird features of the data, while others seem less concerned about them… typical when a new discovery is claimed. I’m disturbed that the media is declaring victory before the scientific community is ready to. That didn’t happen with the Higgs discovery, where the media was, wisely, far more patient.
The Main Data
Here’s BICEP2’s data! The black dots at the bottom of this figure, showing evidence of B-mode polarization both at small scales (“Multipole” >> 100, where it is due to gravitational lensing of E-mode polarization) and at large scales (“Multipole” << 100, where it is potentially due to gravitational waves from a period of cosmic inflation preceding the Hot Big Bang.) All the other dots on the figure are from other experiments, including the original BICEP, which only put upper bounds on how big the B-mode polarization could be. So all the rest of the points are previous non-detections.
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.
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.