Starts With A Bang

Comments of the Week #87: From Quasars to Einstein, and Thanksgiving

Image credit: public domain photo from

“At times our own light goes out and is rekindled by a spark from another person. Each of us has cause to think with deep gratitude of those who have lighted the flame within us.” -Albert Schweitzer

It was an action-packed week here at Starts With A Bang, where we took on all of the following:


There’s so much more that’s on its way, including our second podcast coming next week, an upcoming holiday giveaway for the lucky submissions chosen for Ask Ethan, and some more spectacular articles about how the Universe came to be, and how we know it. (Oh, and we’ve also almost unlocked the next Patreon reward, for real! Let’s make it happen before 2015 is out!)

Now, join me as we jump in to our Comments of the Week!

Image credit: ISS astronaut Scott Kelly, via

From Dean on the “UFO” seen from the ISS: “‘As I explain, however, this is simply light reflecting off of the ISS’s HDEV module, nothing more complex or extraordinary than that.’
That’s what they WANT us to believe.

What’s kind of remarkable is that no one caught my real mistake: it wasn’t the HDEV module at all that caused the reflection, but rather the window shutter to the ESA’s Destiny’s Lab.

Image credit: ISS / NASA, with annotations by Sam Treadgold.

So, I suppose, it could be construed as a case of Europeans pranking Americans, and our tendencies to “want to believe.” This is what we get for having American Thanksgiving, isn’t it, Europe?

Image credit: NASA/JPL-Caltech.

From Gary S on dark matter hairs vs. haze: “Is there some reason to think the (undetectable!) dark matter particles have a [preferred] travel direction near our solar system? If not, then there would be no “hairs” – just a haze around massive bodies with only slightly increased concentration.”

There are two things that I want to respond to here, one of which you’ve realized and one that you haven’t. Dark matter particles are expected to move in a virialized fashion, which means their velocities should be randomly distributed with a gradient towards/away from the center of gravity. The Earth’s motion — around the Sun and through the galaxy — should be added/subtracted from that.

In addition, we don’t think that dark matter should have the same rotational speed around the center that the normal matter does, so Earth should be moving, on average at 190-250 km/s relative to the dark matter.

Image credit: Gary Prezeau, via

But then there’s this part: the focusing part. If there’s dark matter coming in at the same trajectory, which there ought to be, the planet’s gravity will focus that into a stream, and then into a hair. Commenter Wow made an allusion to this when he said:

“Gary, the idea is that this is a similar thing to [gravitational] lensing, where gravity makes a stream of particles bend their path, refracting to a focal point.”

The main difference is that there is a velocity dispersion and an impact parameter range here, which leads to the development of a hair. The important point, though, is that this fluffy, diffuse “cloud” of dark matter will generically give rise to a caustic, or a point where the density spikes incredibly high, the same way that a magnifying glass focuses the Sun’s rays to a point.

Image credit: flickr user Dave Gough, via

This is a generic behavior of cold, collisionless dark matter, even if it’s isothermally distributed. This was shown by Pierre Sikivie in (I believe) 1995, and is accepted and used by astrophysicists everywhere. It’s even referenced in Gary Prezeau’s paper. Sikivie was on my thesis defense committee; it’s amazing how small the world of cosmology truly is.

Image credit: The Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, Calif.

From Gary S on the successor to the Hooker 100″ telescope: ““Mt. Wilson, which then housed a telescope twice as large…”
If you meant the 200 inch, isn’t that on Mt. Palomar?”

The 200″ Hale was on Mt. Palomar instead of Mt. Wilson, you are correct. The 100″ was on Mt. Wilson and Hubble was the primary name associated with both. What’s interesting is the 100″, commissioned in 1917, was the largest in the world until the Hale, with only Otto Struve’s 82″ telescope (in the 1930s) coming close. When the 200″ was commissioned in the late 1940s, it was the largest until the Soviet BTA-6 in 1976 and the second largest until the Keck telescope in the 1990s.

Image credit: Wikimedia Commons user Cmglee.

What I think is remarkable is the advent of segmented mirrors and of telescope arrays. No longer are we limited by what glass can do under the force of gravity, but rather by finances alone. More area = more light = more power, and when it comes to exploring the distant Universe, that means more details, more data, and more knowledge.

Image credit: NASA / JPL.

From Hephaestus on comets and Oort clouds: “People always refer to passing stars disturbing our Oort cloud and launching a comet Sun-ward. What about the Sun disturbing other stars’ clouds and drawing off one or more of their Oortlets? We must be near the edge of the Centauri system’s cloud.”

Don’t let a diagram like this — which is on a logarithmic scale — fool you. The Oort cloud may go out as far as about half a light year, but the gravitational binding is extraordinarily tenuous. When a star comes close by us, which happens every few 100,000 years or so, our Oort cloud objects get perturbed, and so do theirs.

Image credit: Michael Osadciw/University of Rochester.

Yes, these stars should have their own Oort clouds, and the mutual interactions should perturb both our Oort cloud objects and also theirs. But consider the difference between these two scenarios:

  1. 50% of our perturbed Oort cloud objects will lose velocity with respect to our Sun, reducing their orbits and sending them closer to the inner Solar System. A few of these — the ones that lose the most heliocentric velocity — get close enough to the Sun to become comets.
  2. 50% of the other star’s perturbed Oort cloud objects will gain velocity with respect to its Sun, many (perhaps most) of which will be ejected from their solar system. But only a tiny percentage will wind up being directed towards our inner Solar System, and an even smaller fraction will get an additional gravitational interaction that causes them to wind up on a stable (elliptical, rather than hyperbolic) orbit.

So it’s possible, but we can say with 98%+ certainty that each comet we see from the Oort cloud is from our Oort cloud.

Image credit: Keck / UCLA galactic center group / A. Ghez et al., via

From Waterbergs on a black hole’s event horizon: “Does this mean the event horizon of the [black holes] gets squashed out of spherical into some increasingly strange shape? As I understand it [Sagittarius] A* has a mass of about 4 million solar masses. If one assumes each [black] hole contributed around 20 solar masses then this means 200 000 black holes merged to create it.”

The event horizon of a non-rotating black hole is spherical; the event horizon of a rotating black hole is not. What’s really fun here is that the singularity of a non-rotating black hole is a point; the event horizon of a rotating black hole is a ring, or a 1-dimensional singularity rather than a 0-dimensional one.

It isn’t the act of mergers that cause the distortion; it’s the end state, and how much angular momentum it has. The event horizon winds up being ellipsoidal rather than spherical, and more technically like an oblate spheroid. Fun, but counterintuitive stuff!

Image credit: Wikimedia Commons user Brews ohare.

And finally, from Denier as a follow-up on the Lemaître/Hubble controversy: “When originally learning of this story, I seem to remember the reason the critical portion wasn’t translated was because Albert Einstein criticized it. Prior to publication, he shared his finding with Einstein who replied with something to the affect of ‘I can’t find any fault in your math but your physics are atrocious’. Lemaître held Einstein in high esteem, and that scorching rebuke caused Lemaître to doubt his own work. The section wasn’t translated because Lemaître was trying to save himself from further ridicule.”

Einstein said a lot of dismissive, wrongheaded things, particularly after 1915. He derided every new development in quantum physics; he railed against the developing science of particle physics; he constantly returned to his old, discredited ideas; he failed to appreciate the nuance and insight of his contemporaries like Pauli, Schrodinger, Bohr, Dirac and others. In short, Einstein got left behind.

This is very different than my favorite stories from the book Denier refers to: The Very First Light by Mather and Boslough, which really give some insight into how dogmatic Fred Hoyle was. He was so repulsed by the idea that the Big Bang could be right, because it seemed to jive somewhat with a biblical story and because the innovation could be connected to Lemaître, a priest, that he never let a shred of evidence sway his opinion. In short, despite all the developments he initiated in stellar nucleosynthesis and other areas of cosmology and astrophysics, he became one of the saddest stories in all of late 20th century science.

Image credit: E. Siegel and World Scientific.

Those of you who want to learn more about that particular time of development in science will particularly enjoy chapters 4-6 of my book, Beyond The Galaxy: how humanity looked beyond the Milky Way and discovered the entire Universe. It’s available for your holiday shopping now, for pre-order and for 30% off with the code WS15XMAS30 at checkout. Get copies for yourself, and for all your science-minded friends, today!