Comments of the Week #28: The end of the Universe, world, and one wonderful dog

“I took a deep breath and listened to the old brag of my heart. I am, I am, I am.” -Sylvia Plath

It's been two weeks since our last edition of our Comments of the Week, and from the heartbreaking to the mystifying, there's a lot we've written about and explored together. If you missed anything, go ahead and take a look back at our amazing suite of articles over that time:

As always, you've had a lot of fascinating things to say, and I wish I could find the time to answer them all. As it is, I wanted to call out the ones that range from my favorites to the ones I think give the most opportunity for education. With that said, let's dive on in!

Image credit: Scientific American, via Image credit: Scientific American, via

From Paolo Janotti on the topic of the Big Rip: "Testing my understanding: to the best of our knowledge the galaxies are getting farther away faster, i.e.: the universe is expa[n]ding faster, but not fast enough for the big rip. Is that right?"

This is close, but not quite right. When we talk about dark energy and the accelerating Universe, what we mean is that as galaxies get farther and farther away, they appear to move away from us at a faster and faster rate. But is that because the expansion rate is speeding up? Not necessarily. Remember that the expansion rate is a speed per unit distance, meaning that how fast something moves away from us depends on how far away it is. An individual galaxy could still be accelerating in three separate cases:

  1. The expansion rate drops, but at a slow enough rate that as individual galaxies move farther and farther away, they speed away at an increasing velocity. This a Universe where there is continued acceleration, but at a decreasing rate.
  2. The expansion rate remains constant, so as individual galaxies move farther away, they accelerate at a constant rate. Their velocities will continue to increase at a faster rate than the first case, but the acceleration rate never increases.
  3. Finally, the expansion rate increases as time goes on. In this case, not only to the velocities rise, but the acceleration rate increases, too. At some point in time, they all spike towards infinity, and that's where you get a Big Rip.
Image credit: Quantum Stories, retrieved via Image credit: Quantum Stories, retrieved via

All three of these cases involve some form of dark energy, but which one we have depends on whether the dark energy density decreases (1), remains constant (2), or increases (3) as time goes on. As best as we can tell, we live in the first case right now, but at the (both dark and normal) matter density drops, we are asymptoting to the second case. The Big Rip is a possibility, but one that gets pushed out later-and-later as our constraints become better and better. At this point in time, the minimum amount of time to the Big Rip from now is about 80 billion years, or about six times the present age of the Universe.

But if I were a betting man, I'd bet that our long-term future holds the fate of option 2 for us, unless something happens to alter it. This means that individually bound structures -- like galaxies and star systems -- won't ever be torn apart by dark energy, just distant structures that are still expanding away from one another today.

Image credit: R. Brent Tully (U. Hawaii) et al., SDvision, DP, CEA/Saclay, of Laniakea, our local “supercluster” of galaxies. Image credit: R. Brent Tully (U. Hawaii) et al., SDvision, DP, CEA/Saclay, of Laniakea, our local “supercluster” of galaxies.

From Michael Kelsey on the topic of superclusters and large-scale structure: "I’m not sure I entirely agree with your broad statement here, “our best simulations of gravitation … reproduce to arbitrary accuracy the Universe we actually observe …” As I understand it, our best simulations, using the “nominal” LCDM parameters, do _not_ reproduce the Universe we actually observe..."

I should have put in an additional caveat, because the statement is true for the largest scales in the Universe. Want to know what the Universe looks like on the scale of a few million light years? Or a few tens of millions? Or hundreds of millions? Or billions? Or tens of billions? Or on the scale of the entire observable Universe?

Our best simulations of gravitation in the Universe with normal matter, dark matter and dark energy -- as well as the appropriate amounts of radiation and neutrinos -- do reproduce the Universe we observe to arbitrary accuracy on these scales.

Image credit: Richard Powell of, under C.C.-by-S.A.-2.5. This image spans roughly 500 million light-years in radius. Image credit: Richard Powell of, under C.C.-by-S.A.-2.5. This image spans roughly 500 million light-years in radius.

But in the interest of full scientific disclosure, the small-scale structures that Michael mentioned: the individual density profiles of galaxies and the abundance of dwarf satellite galaxies, do represent a significant problem (and opportunity!) for the simplest models of dark matter. If dark matter truly is cold and collisionless, interacting only gravitationally, then perhaps there's new physics involved. But if dark matter either has a non-negligible temperature or interacts either with itself or with normal matter in a non-trivial way, this problem could solve itself. One of the reasons I'm excited that James Bullock is writing for us now is that he's one of the leading scientists working on this approach: using what we see about the smaller-scale structure in the Universe to potentially teach us about dark matter's properties.

Stay tuned!

Image credit: ESO Photo Ambassador Gianluca Lombardi. Image credit: ESO Photo Ambassador Gianluca Lombardi.

From Ross Presser on the science of the green flash: "Should we not see green flashes at sunrise, as well?"

Yes, absolutely! The atmosphere towards the eastern horizon is no different than the atmosphere towards the western horizon in its composition or characteristics, and so if you have clear conditions conducive to observing a green flash, you should be able to see it just the same.

There is, however, one associated difficulty that makes it so that almost all pictures of the green flash are taken at sunset rather than sunrise.

Image credit: Pekka Parvianen in Finland, 1992. Image credit: Pekka Parvianen in Finland, 1992.

Clear skies towards the east seem to be a rarity before sunrise, but what's even harder to get a handle on is what the moment will be just prior to the Sun rising above the horizon at dawn, as compared to what the moment will be just subsequent to the Sun setting below the horizon at dusk. Think about it: it's a lot easier to watch the Sun set and then watch those final moments and the ones just after, than it is to watch the Sun just prior to its rising and capture those moments.

But in principle, the physics -- and the opportunity for a green flash siting -- is the same under similar atmospheric conditions.

Image credit: me, of my first dog, Cordelia, back in 2008. Image credit: me, of my first dog, Cordelia, back in 2008.

From Hans Deuze on the topic of saying goodbye to my dog Cordelia, "Hi Ethan, my sincere condolences and thanks for sharing this with your audience. I think most of us have had the same experience with the pets we had to say goodbye to… While reading it was almost as if Cordelia was my dog. You took the right decision and fortunately we can do this merciful act for our dear beloved friends. I’m grateful I live in the Netherlands where the same is possible for people should they wish to die with dignity."

I want to thank all of you for your outpouring of support and your total lack of "how dare you take time off from writing to spend time with your family after your loss." Class act all the way on all of your parts. I teared up just a couple of days ago when I came home from work and we took Shao May into the yard. When I asked "do you want to come inside?" she of course ran full-bore towards the door, like she always does. But I expected to feel the light little touch of a nose on the back of my left calf, which would have been Cordelia's oh-so delicate way of herding me towards the door. Of course the touch never came, and for some reason that made me miss having her in the world all-of-a-sudden all over again.

Photo credit: Sam from Double Dog Ranch in Rainier, OR. Photo credit: Sam from Double Dog Ranch in Rainier, OR.

But the best comment I got -- and I wanted to share with you -- came on facebook, from a young adult who happened to have a connection to her all those years ago:

"I'm so sorry to hear about Cordelia... As that 14 year old girl, I can't tell you how much I appreciate you both for taking her on and giving her such an amazing life. I'm so grateful she was given the chance to live and that she was lucky enough to find you two when almost any other family would have given up on her immediately. I couldn't imagine a better home for her. Thank you for truly rescuing her. "

Thank you, Kate, and thank you everyone.

From Omega Centauri on the Physics of the Death Star: "So less than one solar mass energy can undue the binding energy of a galaxy? Are you sure?"

From a purely energetic point-of-view, absolutely! From a practical point-of-view, no way. If you get, say, an asteroid's worth of antimatter down to the center of Earth and ignite it (by colliding it with normal matter), it's going to cause a huge explosion that releases energy spherically outward, and that's how you'll destroy a planet. That also explains how a simple proton torpedo (or two, just to be safe) can ignite a chain-reaction that causes the entire Death Star to spectacularly explode: if there's antimatter on board, that spherically-outward explosion is going to literally blow your device apart!

But for a galaxy, it isn't a solid, spherical structure where all the outward flux from an explosion like this could simply drive the matter apart. A single Sun-like star of antimatter would -- assuming it annihilated with an equal amount of matter -- produce enough energy to drive apart about five billion solar masses worth of a galaxy, or all but the three largest galaxies in our local group. But getting that energy to work at overcoming the gravitational binding energy is another story.

Maybe, just maybe, that's what episode VII will be about!

Image credit: ESA and the Planck Collaboration. Image credit: ESA and the Planck Collaboration.

And finally, from Pavel on signatures from the early Universe: "When the CMB formed, the radiation didn’t have enough energy to ionize atoms, but it had (for some time period) surely enough energy to excite atoms. And this excitations and de-excita[t]ions shall leave some traces in both absorption and emission CMB spectra (especially if the exited state decayed by radiating two photons).
Are these traces to faint to detect them? Or is this idea complete nonsense?"

The problem with looking for signatures like this is threefold. You see, it is a good idea in the sense that there are enough CMB photons that these excitations do, in fact happen. And these atoms do then emit photons (the de-excitations) of those particular wavelengths. But we don't see emission line signatures in the CMB for three reasons:

  1. The CMB photons outnumber atoms by a little more than a billion-to-one, meaning that the atomic signature, no matter what it is, will be swamped by the background.
  2. Every atomic photon that's emitted corresponds to a CMB photon that was absorbed. With the minor caveat that there's a little bit of redshift that happens between absorption and re-emission, there's no net change to the CMB.
  3. And finally, the Universe doesn't become neutral all at once, but over a time period of more than 100,000 years; it only peaks at 380,000 years of age.

Even so, there's one fun thing to look for.

Image credit: Jens Chluba, Rashid Sunyaev "Free-bound emission from cosmological hydrogen recombination" A&A, 458, L29 (2006). Image credit: Jens Chluba, Rashid Sunyaev "Free-bound emission from cosmological hydrogen recombination"
A&A, 458, L29 (2006).

Hydrogen atoms admit many different transitions, and could leave small "gaps" in the CMB and add additional "peaks" at higher frequencies. The effects would be very small as a total fraction of the radiation (note the tiny numbers on the y-axis, above), but it's there in principle, and with more sensitive instruments, there's no reason we couldn't detect it. It would just be hard!

And that's it for this edition of Comments of the Week; thanks all!

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