Comments of the Week #122: from distances in the Universe to the dangers of a parent comet [Starts With A Bang]

 “Failure I can live with. Not trying is what I can’t handle!” -Sanya Richards-Ross

It’s been a crazy week here at Starts With A Bang, including a pretty good meteor shower and some other rather remarkable stories. I just recorded a radio show with Dr. David Livingston of The Space Show this past Tuesday, and it’s downloadable or available for streaming right here!

I’ve confirmed with Forbes that yes, they swear they no longer block ad-blockers, and so if you’re running one and it doesn’t let you through, leave a comment for me and I’ll pass it onto my senior editor. Miss anything this week? Check out all the great news:

• How can we see so far away in such a young Universe? (for Ask Ethan),
• Hubble’s bubble illuminates the interstellar rubble (for Mostly Mute Monday),
• How did we fool ourselves into believing in a new particle that wasn’t there?,
• Could you travel in a straight line in space and return to Earth?,
• The most dangerous object known to humanity, and
• The least likely record to fall this Olympics, and beyond.

There’s a lot to look back on, so let’s dive into our comments of the week!

From Denier about crime ticking upwards: “I can show hard evidence from neutral sources showing crime has ticked up and those with the short attention point of view do indeed have a point. I can transcribe text to show the point of a discussion was an opinion over appropriateness of tone rather than denying science. In the end it likely doesn’t matter because even with contrary evidence right in their face, people willfully won’t see it.”

There’s a lot that’s been said already so I won’t go over all of the old stuff, but there’s an interesting thing to consider and digest here: whether short-term “upticks” in the data (or downticks, for that matter) are significant when talking about long-term trends. This is a question that should have a quantitative answer. It’s a question where we should be able to look at things like “how much data do we need to know whether this uptick is meaningful or not?” You know, like we do with the climate science data.

The conclusion for the climate data, by the way, is that if one wishes to pull out an accurate warming trend, one must take data sets in increments no smaller than 15-to-17 years. If crime is ticking upwards over a 3-month or 6-month period, how big is that uptick and how significant is it? Does this affect the overall trend, or is it consistent with a random fluctuation?

But perhaps it’s only my opinion that asking scientifically meaningful questions like this is important when it comes to the data. Perhaps Newt is right, that when it comes to politicians and what garners them the most positive attention, it’s much more about how people feel about what’s happening than what’s actually happening. Taking the cue from climate science, that appears to be absolutely the case. Maybe Kipling had it right when he wrote his famous epitaph for a dead statesman:

From Michael Kelsey on distances in the Universe: “How do we _know_ the distances so far beyond standard candles? It seems to me that we get the redshift distance from z, and from that the redshift age. But converting redshift age into “true comoving distance” involves making the GR assumption. Is there any obvious observational way to independently confirm the GR curve? Is that what “angular diameter distance” does for us?”

You do have to make the GR assumption at some point, but only in the sense that there are different ways of measuring distance that depend on what the fabric of space has been doing. As the Universe expands, different distance measures depend on the redshift in different ways. If you measure the angular size of an object, for example, the angular diameter distance depends on the comoving distance divided by (1 + z). If you measure the brightness (luminosity) of an object, thought, the luminosity distance depends on the comoving distance multiplied by (1+z).

Above, you can see the luminosity distance, the “naive Hubble” distance (just given by v/H), the comoving distance (different from the Hubble estimate) and, at bottom (skipping lookback time), the angular diameter distance. We have multiple methods that we can use on the same galaxy, and we get different results. In a non-GR Universe, particularly at high redshift, a well-understood single object is enough to falsify the notion that all these different measures yield the same answer.

From PJ on the Bubble Nebula: “This looks like a long term pregnancy!”

I had never, ever thought of that before. I had called these new star-forming regions stellar nurseries before, but yes, the bluish coloration makes it look like it’s filled with amniotic fluid and that there’s some kind of stellar fetus inside.

Just keep in mind, at some point — perhaps while the bubble is still present — this star will go supernova. But that’s not a cosmic birth; it’s an incredible cosmic death. Does that make this a cosmic miscarriage?

From Denier on invisible particles: “If the LHC was creating new particles other than the ones already in the Standard Model, but those particles were sterile, would we know?”

When you make a new particle in a collider, you have to conserve both energy and momentum. Most collisions between particles are glancing, meaning each one maintains almost all of their original momentum. This is just fine, because we build our detectors with the intent of detecting particles with a large transverse momentum, meaning we focus on the rare collisions that not only hit each other head-on, but where a significant amount of the momentum happens to be transverse to the beam direction. This is why detectors have the shapes they do.

There are a few different types of calorimeters inside to measure charged particles and their energies as well as photons; there are magnetic fields and ionization trackers to measure charges and momenta; there are the outer layers to measure muons, and then there’s “missing energy.” If we reconstruct both the missing energy and the missing momentum, we can determine whether these are light, ultra-relativistic particles like neutrinos, or whether they have a good sized rest mass and are sterile “other” particles, like WIMP dark matter. So far, we don’t have any sterile particles, but those simple tasks — measure the energy and momentum of everything that comes out and look for what’s left over — will give you the mass of a missing particle.

Image credit: The Marenostrum Numerical Cosmology Project, with acknowledgment to Arman Khalatian and Klaus Dolag.

From See Noevo on dark stuff: “What is the sigma up to for the existence of dark matter and dark energy?”

This depends incredibly strongly on which component of the data set you’re looking at. If you look at interacting galaxy pairs, you get about a 3-sigma result. If you look at the rotation curves of an individual galaxy, you get around 5-sigma. If you look at supernovae for dark energy, you get 7-sigma. And if you look at single colliding galaxy cluster pair and the weak lensing data they provide, you get a 12-sigma result.

But that’s not the way we do science. We don’t cherry pick one set of results; we look at the full suite! That means taking the best data sets from all available, reliable sources and combining them together to draw your conclusion. This has been done, as Michael Kelsey notes! If you look at the weak lensing, BAO, supernova and CMB data combined:

Baryon density (Ωb): 0.0486±0.0010
Dark matter density (Ωc): 0.2589±0.0057
Dark energy density (ΩΛ): 0.6911±0.0062

Which is to say, dark matter has about a 45-sigma significance, and dark energy has over a 100-sigma significance. This should impress anyone except the most fervent ideologue.

From eric on returning to your starting point in the Universe: “Ethan takes on the case of whether you can do this in the current universe, but we can also consider it as a challenge to human engineering. I.e., can we build a curved space such that we can walk in a straight line and arrive back where we started? Here’s how.
Step 1: build a wormhole.
Step 2: walk in wormhole until you’re at the middle (well, at least away from the edge).
Step 3: make a left (or a right) 90 degree turn.
Step 4: walk in straight line.
You will arrive back where you started once you have traversed the circumference of the wormhole.”

Ahh, the good old timelike geodesic for a slower-than-light observer. This is the lamest way to come back where you started; you could also get there by building a precise rocket that:

1. launched you up into space,
2. at speeds too slow to enter low-Earth orbit,
3. so you’d fall back down to the planet’s surface,
4. but calculated so precisely so that you hit your launch location exactly.

But this is lame! This is a cop-out. If you want to really do this justice, you should be moving at the speed of light; you should be a null geodesic!

And as for black hole/wormhole/gravitational slingshots, that’s a cop-out, too. You should be looking to circumnavicate the Universe, and do this for real. Just as you could walk in a circle of any arbitrary size and claim “I went around the world!” and your skeptic friends could punch you for lying about the obvious meaning of “around the world,” you could take a shortcut through the Universe to arrive back at your start point. But what’s the point? Come out one side of the Universe and back inside the other, like the proposition intended!

Sure, with dark energy, you might need a warp drive to do it, and I can support that kind of cheating. But defeating the premise and not learning whether the Universe is even topologically closed or not? That’s a sad, cheating story.

And finally, from Omega Centauri on comet Swift-Tuttle, and the energy it contains: “Ethan, I think you dropped a zero. In comparing kinetic energy versus the C-T impactor 2.6**3 *4**2 = 280, not 28.”

There is actually a much more sophisticated way to calculate the energy of a potential impactor, and I didn’t go into the details in the post, because… well, it’s a pain in the neck! But for you, Omega, let’s do it! The biggest difference between what the KT impactor was composed of (in assumptions) and what Swift-Tuttle appears to be is in terms of density. To calculate this, I used the traditional method given by Weissman, articulated here: http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521863452

A radius of ~13 km and an estimated density of 600 kg/m^3 gives a cometary mass of ~6×10^15 kg. This is, interestingly enough, about an order of magnitude lower than an asteroid’s density, which should be closer to 4,000-5,000 kg/m^3. An impact velocity of 61 km/s, which is approximately (3.8-4.0) times the impact velocity of the KT impactor yields an impact energy of 2.75×10^9 megatons. The dino killer was estimated at ~10^8 megatons, and so there’s your factor of 28, not 280.

I’m actually on vacation, but you’d never know because I couldn’t help but make sure there was no lag or drop-off in these articles for you, my dear readers. I’ll read everything you say when I get back on the 21st, but in the meantime, enjoy the rest of what the Universe brings to you!