“We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.” -T.S. Eliot
It’s been a remarkably exciting week for science, and you’ve had a lot to say about our new articles at the main Starts With A Bang blog. After all, it isn’t every week that we possibly discover the imprint of gravitational waves on the Cosmic Microwave Background!
Let’s dive right in — and respond — to your comments of the week!
From Michael Kelsey on the topic of Ask Ethan #28: Feeding the Monster: S-2 already made a pericentric pass in 2002, didn’t it? Has the technology improved enough that we will learn substantially more this time than last? Or is it just that we have better focused questions to ask, and can therefore choose particular instrumentation to use?
If you’re looking at the supermassive black hole in the center of our galaxy, you won’t find any light coming from it. But if you look in the infrared, you can find the light coming from stars that orbit it! Pay close attention to the one labeled “S0-2” in the animation, below. (It’s also known as “S2“, for “source 2”.)
With an orbital period of just under 16 years around our supermassive black hole, and an eccentric, elliptical orbit to boot, it has, in fact, made one close pass to our galaxy’s central mass source, back towards the end of 2002. There’s another star, discovered just two years ago — S0-102 — that has an even shorter period orbiting the galactic center. The thing is, S2 is incredibly bright (it’s a B1-class star), it makes a very close approach to this 4.3 million solar mass object (at just four times the Neptune-Sun distance, or 120 AU), and at the moment of its closest approach, it will be moving at around 5,000 km/s, or nearly 2% the speed of light.
In other words, this is the perfect regime for observing relativistic effects! We did, in fact, learn a lot from the 2002 pass, but you must realize that instruments improve a lot in 16 years! We won’t have the James Webb Space Telescope online quite yet, but now that the orbit is precisely determined, we can look for the relativistic redshift during closest approach, something that Keck and the VLT combined are expected to measure to 5-sigma in 2018.
So, both: we have better instruments (although the JWST and the proposed Thirty-Meter-Telescope could do much better than present technology), but we also know what to look for to very high precision now!
On average, the Earth takes about 58 days to traverse the equivalent of the Earth-Sun distance, and on average, particles ejected from the Sun take about 3 days to reach the Earth. In other words, the Solar Wind travels more than ten times as fast as the Earth orbiting the Sun.
Am I missing something here? Last time I did any classical mechanics, forces exerted as a result of orthogonal flows are pretty much independent. In other words, no matter how fast the solar wind is moving, the orbital drag is a function of orbital velocity and the density of the matter swept up by the Earth as it goes through the neighborhood.
Otherwise, I don’t see how you’d conserve angular momentum.
There’s a funny thing about orbits, relevant to particles ejected by the Sun colliding with our planet. Imagine what would happen if you threw a piece of putty outward, from the Sun, and it collided with the Earth.
When you think about something colliding with our Earth, you must remember this is an inelastic collision, and that even though its linear momentum might be perpendicular to the direction of our orbital motion, the combined system at the end is either going to have greater kinetic energy-to-mass ratio or a smaller one after the collision.
If we wind up with a greater amount, we’ll be more loosely gravitationally bound, and therefore will experience an increase in our semimajor axis, while if it’s lessened, we’ll be more tightly bound, and will wind up orbiting closer. It’s not a flow pushing us out, it’s a series of sticky particles colliding with us, giving us an extra, energetic kick and pushing us into higher-energy orbits.
That’s why we’ll spiral outwards as our Sun progresses to (and lives in) its red giant phase!
From Michael Fisher concerning the globular cluster Messier 68: Such globular clusters that are low in elements above lithium… Little chance of Earth-size rocky planets? Would stars in these clusters lack outer regions [sort of beyond 1.5 AU] containing water ice, comets, asteroids & other orbiting objects of interest?
Globular clusters are typically ancient and low in heavy element concentrations; with less than 1% the amount of heavy elements our Solar System has, the stars in this one are among the most metal-poor ones in the local Universe!
But how little is too little to form rocky planets, or to have a frost line in a star system? How dense and chaotic an environment is too dense or chaotic to have a Kuiper Belt or Oort cloud around a system? We don’t have sufficient observational evidence to know where those lines are, but 1% of what we have here is still quite significant! Consider that our Moon is just 1.2% the mass of our Earth; if we had a Moon-sized object orbiting a star in the inner Solar System, we’d certainly consider it a planet, wouldn’t we? Consider that Jupiter is 318 times the mass of Earth; wouldn’t it be conceivable to get an Earth-mass planet in that system?
My hunch is that in a loose globular cluster like this, we would have both Earth-sized planets and — for many of the stars inside — frost lines and abundant comets. (Although, I also wouldn’t be surprised if overlapping Oort clouds didn’t lead to a spectacular cometary show early on in the life of a globular cluster, and that they’d be much rarer today…)
From Algernon concerning All About Cosmic Inflation: do we actually have quantitative estimates about the inflation epoch – i.e. size of the universe when it started/ended, time when it started/ended, energy scales involved, etc.? E.g. where does that 10^-32 seconds come from…?
So if you read and understood what I was writing about, you probably are even more confused about (incorrect) timelines like the one the NSF shows about the history of the Universe.
Why do I take issue with this? If you naively extrapolated backwards to an infinitely hot, dense state, you would have had a singularity at the time “t=0,” assuming that today the age of the Universe is our “t=13.8 billion years.” But if cosmic inflation happened and set up the Big Bang, this is no longer true. In particular, there’s no longer a singularity, and there’s no longer any special meaning to “t=0”, other than the fact that the part of the inflationary epoch whose gravitational waves and density fluctuations are stretched across our Universe today came from those first 10^-30something seconds.
Where did inflation come from? How long did the inflationary epoch last? Was there a singularity before inflation got its start, or was inflation eternal to the past?
We don’t know, and by the very nature of inflation — by the fact that it wipes out whatever information was present in the Universe before the final 10^-30something seconds of inflation and its transition to the hot Big Bang — we may never be able to answer those questions, because that information doesn’t exist in our Universe. But that’s why I think it’s so important to be correct and precise about what we do know.
And our final question…
From Anne Blankert as respects The Cosmic Speed Limit: Besides the concept of fixed maximum speed, another concept of relativity is that speed can only be measured (exist) relative to something else. Is the cosmic background radiation some sort of fixed reference frame so that it will fry us when we speed up relative to that?
The cosmic background radiation is the radiation that the Universe emits — or rather, that free-streams from a surface of last-scattering — when electrons and atomic nuclei finish combining to form neutral atoms. It comes to us in all directions, with a uniform temperature of around 2.725 Kelvin, that continues to drop as time goes on.
But this doesn’t pose a problem for relativity at all; on the contrary, in the context of general relativity, we can use the temperature imperfections in the cosmic microwave background to figure out what our own peculiar velocity is with respect to the Hubble flow. We find — just as we expect — that the light from the CMB is redshifted slightly in one direction, and blueshifted slightly in the opposite one.All told, we have a cosmic motion of around 670 km/s relative to this rest frame. But if we wanted to fry ourselves, we’d need to be moving awfully fast; even moving through the Universe at 99.99999% the speed of light would only turn the CMB into the energy equivalent of sunlight, so we’ve got a long way to go before we do anything that fries us!
Hope you enjoyed this edition of Comments of the Week, and looking forward to some good ones for next week!