Nothing goes faster than the speed of light in empty space, also known as the cosmic speed limit c. Right? Well, umm… the devil is in the details.
Here are some of those details:
(more…)Tag: inflation
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, including a great deal of background information, click here.] The BICEP2 scientists and the Planck satellite scientists have worked together to come to this conclusion, and written a joint paper on the subject. Their conclusion is that the potentially exciting effect that BICEP2 observed (“B-mode polarization of the cosmic microwave background on large scales”; these terms are explained here) was due, completely or in large part, to polarized dust in our galaxy (the Milky Way). The story of how they came to this conclusion is interesting, and my goal today is to explain it to non-experts. Click here to read more.
POSTED BY Matt Strassler
ON February 6, 2015
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 of their work (see also here).
- There is relatively little doubt (but it still requires confirmation by another experiment!) that BICEP2 has observed interesting polarization of the cosmic microwave background (specifically: B-mode polarization that is not from gravitational lensing of E-mode polarization; see here for more about what BICEP2 measured)
- But no one, including BICEP2, can say for sure whether it is due to ancient gravitational waves from cosmic inflation, or to polarized dust in the galaxy, or to a mix of the two; and the BICEP2 folks are explicitly less certain about this, in the current version of their paper, than in their original implicit and explicit statements.
And we won’t know whether it’s all just dust until there’s more data, which should start to show up in coming months, from BICEP2 itself, from Planck, and from other sources. However, be warned: the measurements of the very faint dust that might be present in BICEP2’s region of the sky are extremely difficult, and the new data might not be immediately convincing. To come to a consensus might take a few years rather than a few months. Be patient; the process of science, being self-correcting, will eventually get it straight, but not if you rush it.
Sorry I haven’t time to say more right now.
POSTED BY Matt Strassler
ON June 20, 2014
The controversy continues to develop over the interpretation of the results from BICEP2, the experiment that detected “B-mode” polarization in the sky, and was hailed as potential evidence of gravitational waves from the early universe, presumably generated during cosmic inflation. [Here’s some background info about the measurement].
Two papers this week (here and here) gave more detailed voice to the opinion that the BICEP2 team may have systematically underestimated the possible impact of polarized dust on their measurement. These papers raise (but cannot settle) the question as to whether the B-mode polarization seen by BICEP2 might be entirely due to this dust — dust which is found throughout our galaxy, but is rather tenuous in the direction of the sky in which BICEP2 was looking.
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:
POSTED BY Matt Strassler
ON May 30, 2014
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.
In my “History of the Universe” article [see also my related articles on cosmic inflation, on the Hot Big Bang, and on the pre-inflation period; also a comment that the Big Bang is an expansion, not an explosion!], the following figure appears, though without the colored zones, which I’ve added for this post. The colored zones emphasize what we know, what we suspect, and what we don’t know at all.
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.
POSTED BY Matt Strassler
ON March 26, 2014
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. (more…)
POSTED BY Matt Strassler
ON March 21, 2014
Well, yesterday was quite a day, and I’m still sifting through the consequences.
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.
But in this post, I’m going to assume assume assume that BICEP2’s results are correct, or essentially correct, and are being correctly interpreted. Let’s assume that [here’s a primer on yesterday’s result that defines these terms]
- they really have detected “B-mode polarization” in the “CMB” [Cosmic Microwave Background, the photons (particles of light) that are the ancient, cool glow leftover from the Hot Big Bang]
- that this B-mode polarization really is a sign of gravitational waves generated during a brief but dramatic period of cosmic inflation that immediately preceded the Hot Big Bang,
Then — IF BICEP2’s results were basically right and were being correctly interpreted concerning inflation — what would be the implications?
Well… Wow… They’d really be quite amazing. (more…)
POSTED BY Matt Strassler
ON March 18, 2014
[For your reference if you can’t follow this post: My History of the Universe, and a primer to help you understand what’s going on today.]
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.
POSTED BY Matt Strassler
ON March 17, 2014
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. (more…)
POSTED BY Matt Strassler
ON March 17, 2014
I haven’t written in detail about the history of the universe before, but with an important announcement coming up today, it was clearly time I do so.
Let’s start from the beginning. How did the universe begin?
You may have heard that “the Big Bang theory says that the universe began with a giant explosion.” THIS IS FALSE. That’s not what the original Big Bang Theory said, and it’s certainly not what the modern form of the Big Bang Theory says. The Big Bang is not like a Big Bomb. It’s not an explosion. It’s not like a seed exploding or expanding into empty space. It’s an expansion of space itself — space that was already large. And in the modern theory of the Big Bang, the hot, dense, cooling universe that people think of as the Big Bang wasn’t even the beginning.
How did the universe begin? We haven’t the faintest idea.
That’s right; we don’t know. And that’s not surprising; we can trace the history back a long way, an amazingly long way, but at some point, what we know, or even what we can make educated guesses about, drops to zero.
Unfortunately, in books, on websites, and on many TV programs, there are many, many, many, many, many descriptions of the universe that say that the Big Bang was the beginning of the universe — that the universe started with a singularity (one which they incorrectly draw as a point in space, rather than a moment in time) — and that we know everything (or can guess everything) that happened after the beginning of the universe. Many of them even explicitly say that the Big Bang was an explosion, or they illustrate it that way — as in, for instance, Stephen Hawking’s TV special on the universe. [Sigh — How are scientists supposed to explain these ideas correctly to the public when Stephen Hawking’s own TV program shows a completely misleading video?!] This is just not true, as any serious expert will tell you.
So what do we actually know? or at least suspect?
Out of the fog of our ignorance comes the strong suspicion — not yet the certainty — that at some point in the distant past (about 13.7 billion years ago) the part of the universe that we can currently observe (let’s call it “the observable patch” of the universe) was subjected to an extraordinary event, called “inflation”.
We suspect it. We have some considerable evidence. We’re looking for more evidence. We might learn more about this any day now. Maybe today’s our day.
Stay tuned for the announcement of a “Major Discovery” out of the Harvard-Smithsonian Center for Astrophysics later today. And then stay further tuned for the community’s interpretation of its reliability.
POSTED BY Matt Strassler
ON March 17, 2014
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