Layout of the universe - Astronomy

I've been working on a space game in my spare time, and lately I've been thinking on how to lay out the universe. Though I've searched around and found it hard to get a good view of what the universe looks like. I did find a lot of good sources, though I'm a very visual learner and found it hard to get a good picture of the universe with what I found. I also don't know what the good, reliable, accurate and up-to-date sources were.

So, are there any good visual sources to get a picture of the universe? Preferably a TV-series or a very well illustrated book, or perhaps even computer program.

Edit: I'm looking for a more general view of the universe. Like what do galaxies look like, and what is their scale. As I understand they contain a combination of planets, solar systems, star clusters, ... Though I couldn't get a good picture of how dense they are. Is there a lot of space (relatively) between these solar systems and star-clusters. I found that our solar system is about 4 light-hours big, though are the neighbouring ones average light years or thousands of light-years away? And in the space between them, is it mostly empty or is there space dust?
The same question for whatever our galaxy circles around (if it does) and what is between different galaxies. (And perhaps what else is there of galaxy-size out there?)

I got a vague view of all of this from reading texts, though I feel like I'm missing a whole lot.

Edit 2: I'm not looking for properties of different planets, that's on a too small scale. In the game I'm planning on making a fictional universe (perhaps randomly generated, though not yet decided), but I want the broad layout to be realistic.

Edit 3: I just found this: http://htwins.net/scale2/ This helped me A LOT in getting a sense of the relative size of things.

Thanks a lot!

the moon - Multiple apparent lunar eclipses last night

I saw something very strange last night that I'm hoping someone can help me explain.
I was sitting on my roof with a night-cap at about 3am last night. The moon, about the last quarter, was low in the sky in the east. Something truly strange happened. The moon started to disappear. There were clouds in the sky, but it wasn't this, I'm quite sure. The moon was being eclipsed. The dark part (at the top of the moon, as the moon was lying on its back - a sailor moon? That's what it looked like, like you could sit in it.) grew, the crescent closed, until it was just a spot of cream colored light and then winked out entirely.

I was a bit amazed - I'd stumbled onto my roof in the dead of the night and by complete chance witnessed a total lunar eclipse? I sat there in awe, staring. That's when the moon started to reappear, in the reverse procedure as its disappearance. Becoming a gleaming point of light, then a thin crescent, then the quarter moon which I suppose is what it is "supposed to look like."

AND THEN IT HAPPENED AGAIN. The moon was eclipsed and revealed again.

Both times the whole process took about 6-10 minutes. I'm not sure if this cycling continued, it was late and I had to sleep. I checked the NASA site for eclipses for my time and location, but no.

Has anyone ever seen anything like this, or have any plausible explanation as to what is going on here?

lagrange point - What is a "jumping trojan"? And what do their orbits look like?

Are their orbits circular as Neptune's (in this case) or highly
eccentrical?

L4 and L5 stability is rather narrow

Source: http://ccar.colorado.edu/asen5050/projects/projects_2010/singh/

A more elliptical or eccentric orbit in resonance with a planet with a circular orbit would reach well outside of L4 and L5 and wouldn't be an L4 or L5 object. (though I have to admit, a planet's eccentricity and the effect that has on L4 and L5 stable zones has gotten me curious), but they would likely need to be pretty close to the same eccentricity or it wouldn't be L4 or L5.

Earth's curious orbital partner 3753 Cruithne probobly isn't stable, long term anyway. It's elongated as you describe and it could be stable for a long time, perhaps millions of years but it's not an L4 or L5. 3753 Cruithne has an orbit around the sun of 364 days, so it, in effect, laps the earth every 364 years, rarely coming close enough to hit or significantly change it's orbit cause it's on a different orbital plane.

For an object go from L4, past L3 into L5 it would need to be orbiting slightly faster than the planet.

Now, that object oscillates between L4 and L5, that would be more exotic and much harder to explain. But passing though L3 is possible if the object is orbiting slightly slower or slightly faster than the orbit of the planet. It could be a Jupiter or other planet's resonance that's causing it, but that's just a guess.

How could an object in the same orbit as Neptune have another orbital
period than Neptune? Or how else can it migrate between L3, L4 and L5?

Saying an L4 or L5 object has exactly the same orbit as the planet is probobly inaccurate. The planet tugs on the L4 or L5 object anyway, so there's kind of a dance with Trojan objects. It might average out to being the same orbit, but it's never precisely the same on any given pass. Tugs from other planets affect the dance too. Jupiter, Saturn and Uranus all probobly tug on Neptune's Trojans, throwing off the clockwork a bit.

And are "trojan" objects expected to gather in L3 too?

Gather, no. Pass through, yes. L3 isn't stable, but that doesn't mean an orbit couldn't pass through L3 and be in a near L3 Orbit for several rotations around the sun, but it's not stable. An L3 orbit would tend to move away from L3. L1, L2 and L3 aren't stable, but they are still good points to put a satellite because less energy is needed to keep a satelite in those regions.

Source: http://map.gsfc.nasa.gov/mission/observatory_l2.html

Hope that's not too clumsy an answer. My layman's attempt. I can try to clean up if needed.

Edit / Update

2010 EN65, per your original link is a co-orbital with Neptune, so it's not a 3/2 or 2/3 or any other fraction it's 1 orbit to 1 orbit.

It's also a Centaur, which means it's long term orbit is not stable. A true Trojan asteroid is stable.

I found an article on Jumping Mechanisms of Trojan Asteroids here - it gives a brief summary but it's a pay article: http://link.springer.com/article/10.1007%2Fs10569-015-9609-4

and a map of EN65's orbit (has to be calculated, it's not been observed for nearly that long), but map below:

Source: http://inspirehep.net/record/1190480/plots#2

It's pattern looks similar to some of Earth's co-orbital asteroids, including 3753 Cruithne that I mentioned above.

diagrams here: http://223.252.18.210/petercaspari/bdi/coorb.htm

So, that's probobly a more accurate comparison than Hildas. Look at 3753 Cruithne, or other planet's co-orbitals.

What confuses me - and I hope you forgive me for only enough to be dangerous, as they say, but what confuses me is that your original link says it's an elliptical orbit but it also seems to be drawn to both L4 and L5, spending more time there, so I might have been incorrect in suggesting the elliptical orbit wouldn't be possible. - sorry bout that. At least, some additional eccentricity seems likely, but I'm guessing, probobly not a huge amount. If I was to guess, I'd guess that EN65 is less eccentric compared to Neptune than 3753 Cruithne is compared to Earth.

Here's a cool picture of Jupiter's Trojans (Jupiter's the King of Trojans, it's got like 50,000 in both L4 and L5)

and you'll probobly notice, there's one point at (or, perhaps more accurately, passing through) L3.

gravity - Do planets repel?

Gravity does not have polarity, it only attracts.

An analogy with magnetic or electric fields is appealing (because they are all field forces, decay with the square of the distance, etc...) but science is not made of analogies, it is made of observations. And no one has observed gravity repulsion.

Gravity is in fact much much weaker than the electric force (the factor has 42 zeros), and the only reason we feel gravity is because there is no repulsion, so all those little tiny mass pulls add up to something sensible. Electrical forces, albeit much stronger, usually cancel each other out due to an equilibrium of positive and negative charges.

This is of course, as far as we have seen in nature.

There is however, a very interesting speculation about what should happen with anti-matter. Would it repel "normal" matter?

In the physics page there is a more in deep answer using the results of quantum.

Recommended reading: Feynman Lectures: Theory of Gravitation

telescope - Enhanced Star-Gazing with Special Glasses

On Murdoch's Mysteries, the technophile detective at the end of the Victorian era made, in one episode, night vision goggles that used mirrors surrounding the eyes, essentially a pair of reflecting telescopes with no magnification.

It would not cover the same angle, though: just straight ahead. The naive design you might throw together would handle light arriving parallel to straight forward, so the actual field of view may or may not be useful. To purposfully make a field of view of a specific size and quality would require knowing something about it.

A common SLR photography lens has an enormous opening compared to your pupil, and high quality formerly pro lenses for pure mechanical film cameras can be found cheaply. Some designs even have a lens board for mounting, rather than a fixed camera enclosure.

In their original use, they project the image on a large (relative to your eye) screen, so produce an image dimmer than life. However, if instead of making a projection 44mm in diameter (say), you added an objective lens made for telescopes or a handy jewler's loupe (any magnifying glass works, but for more than 5× you need a fancy compound lens), you could project all the light into your eye.

Update: using on-hand materials it takes 3 pieces, not 2, and is unacceptably long (unless you want it looking like an old seafarers telescope). To concentrate the entire collected light, the next lens needs to be very large around to scoop in all of it. a hand-magnifier doesn't result in an image as large as the directly-observed target, and a powerful loupe is not large enough.

In the end what is the ultimate matter/element in the universe, due to fusion process in stars?

If we don't wait for too long, where we don't know for sure, whether protons or atomic nuclei stay stable forever, there won't be just one single element.

The most abundant element in the universe will probably stay hydrogen, since it will stay in the intergalactic medium, and thin out by accelerating cosmic expansion.

The second-most abundant element will probably stay helium, mostly for the same reason.

The most frequent stars are probably red dwarfs.
They'll burn hydrogen and helium to oxygen and carbon, and end up as white dwarfs.

Only a small fraction of stars is large enough to burn oxygen and carbon to silicon and iron.

Very heavy elements, like uranium and beyond, as formed in supernovae, will decay to lead, helium (alpha particles), some hydrogen (via neutrons decaying to hydrogen), and a couple of less abundant elements formed by spontaneous fission. Other stable (isotopes of) elements heavier than iron are also formed in, or a consequence of, supernova explosions.

Part of the dead stars will collide, and merge to heavier stars, which will end as intergalactic dust (mixture of elements, e.g. iron and silicon), including planets, after supernova explosions, and as Black Holes. Others will be scattered out of reach of the supermassive black hole in the center of the galaxy (gravitational relaxation).

What is a 'hybrid' eclipse?

A hybrid eclipse is one in which it is a total eclipse in some places and an annular eclipse is seen in other places.

As you may know, a total eclipse is one in which the moon completely covers the disc of the Sun; this allows the solar corona to be seen. Usually the central disc of the Sun is so bright that the corona cannot be seen. An annular eclipse is one in which the Moon is not quite "large" enough to completely block out the Sun - instead, a ring of the Sun's central disc can be seen around the Moon. This ring is too bright for the corona to be seen. Photos of both eclipse types appear below.

Annular Eclipse (credit: Fred Espenak at www.mreclipse.com):

Total Eclipse (credit: Fred Espenak at www.mreclipse.com):

The type of eclipse is determined by the geometry of the Sun, Earth, and Moon. Since the Sun and Moon are not constantly at the same distance from the Earth, their apparent sizes will differ. When the Moon is closer to the Earth it appears larger, and when the Sun is farther away it will appear smaller. For this particular eclipse, the geometry is such that the western part of the eclipse's center line will see an annular eclipse, but the rest will have a total eclipse.

Which wavelength is the most quiet, for a ground-based radio telescope?

This might seem like a backwards question, but I'm interested in what wavelength to select in a (hypothetical) ground-based radio telescope observation to expect detecting as little as possible! :)

It should still be reachable from space, i.e. a wavelength with a high atmospheric absorption is probably a bad candidate.

I guess this could be answered by looking at a broad averaged spectrum collected by a radio telescope, and comparing against an atmospheric absorption spectrum chart.

What effects do other planets have on the solar system?

The gravitational pull of all the planets and the sun, and the rest of the galaxy and the universe, all play a part, but gravitational effects fall off with distance.

For Earth's orbit, the Sun is far and away the single biggest influence. Jupiter perturbs our orbit slightly, but with it or without it we have a simple elliptical orbit round the sun. We can measure the perturbation in our orbit from all the planets, but the effects are minor.

When it comes to gravitational effects on small bodies passing one of the planets, however, the effects can be dramatic - an asteroid passing close to Jupiter will be moved far from its original path. The same would be true when passing close to Saturn or in fact any planet - just to a lesser degree.

So while you can't say that Jupiter protects Earth from anything, especially not the Oort Cloud, you also can't say that it draws objects in. Anything out in the Kuiper belt or Oort cloud will be orbiting the barycentre of the solar system (which is within the Sun) and will keep doing that unless knocked out by something like a collision with another object, or the pull of other Kuiper belt objects like Pluto, which can give a much greater tug.

planet - Planetary gas giants

Short answer: 1) Yes and no; 2) Yes, there is a supercritical fluid, of hydrogen.

Long answer:

It's fairly hot deep down in Jupiter; estimates range from 10,000 K to 24,000 K. You would think that anything in the core would be liquefied, and you could be right. Many models predict that Jupiter's core is rocky, but others predict that it is liquid. Still others say that it is "liquefying" - i.e. parts are liquid and parts are solid. We don't know for sure which are right.

In the early solar system, Jupiter would indeed have to have had a large core of rock (and potentially ice). It would have accreted gases, and eventually the planet we know today would have formed, with thick cloud layers complete with hydrogen, ammonia, and lots of other elements. But after Jupiter accumulated its mantle, there would have been a lot of pressure on the original core. It seems like much of the matter inside it would have been liquefied and carried away ("redistributed") into other parts of the planet. Still, some might have remained.

Surrounding the core, there is a layer of liquid hydrogen. Yep, liquid hydrogen, just as you said in your question. Temperatures and pressures are so high that hydrogen has passed its critical temperature (33 K) and become a supercritical fluid.

So, in summary, we don't know for sure the state of Jupiter's core is, but parts or all of it may be liquefied, and we also think that there is indeed a layer of hydrogen as a supercritical fluid surrounding the core. The same may also be said for other gas giants, although, still, we don't know for sure.

I hope this helps.

Sources:

http://www.dailygalaxy.com/my_weblog/2013/02/jupiters-dissolving-core-sheds-light-on-alien-planets.html
http://www.universetoday.com/47966/jupiters-core/
http://www.nasa.gov/audience/forstudents/5-8/features/what-is-jupiter-58_prt.htm
http://phys.org/news/2011-12-jupiter-core-liquefying.html
https://en.wikipedia.org/wiki/Jupiter#Internal_structure
http://arxiv.org/pdf/1111.6309v1.pdf (You might find this particularly interesting, as it considers gas-giant exoplanets)

black hole - In regards to the holographic principle, what exactly is information?

The answer here is deceptively simple, and has very little to do with gravity directly. You only really need to know a bit of quantum mechanics, and the answer comes almost for free.

In any quantum mechanical theory, we have a Hilbert space $mathcal H$. (Actually, we really want a rigged Hilbert space, but this distinction isn't particularly relevant here.) This can be as simple as the space of a single q-bit, or it can be as complicated as you need it to be (e.g. the Fock spaces which arise in quantum field theory). The Hilbert space describes all the possible configurations of any system described by this theory; an individual vector is a specific configuration. We also have a linear operator $H$ on $mathcal H$, called the Hamiltonian, which describes the dynamics of such a system via the Schrödinger equation.

The "information" is just knowing what state $| psi rangle in mathcal H$ your system is in. This state vector tells you the result of every possible measurement, and thus contains all the information of your system. If you really wanted to, it's possible to express any such state uniquely as a sequence of zeros and ones, but that's a very classical way of thinking, and we're dealing with quantum information, which means that the fundamental objects aren't zeros and ones, but state vectors.

So, when we talk about holography, what we're really saying is that we can determine the state $| psi rangle$ that our system (e.g. the universe) is in simply from knowing the results of experiments we perform on the boundary of the spacetime (e.g. infinitely far away). The holographic principle alone doesn't say how this reconstruction works, only that it is possible.

Since this is a bit broad, it may be helpful to see how it works in the only really well-understood example, the AdS/CFT correspondence. In this case, we have a theory of quantum gravity in $d+1$ dimensions with Hilbert space $mathcal H$ and Hamiltonian $H$, constrained to have spatially-asymptotically AdS$_{d+1}$ metric (AdS is just a special, maximally symmetric solution to the vacuum Einstein field equations which has nice properties that make this work). It turns out that, at least morally speaking (there is ongoing work to understand the full extent of this), when we collect all the observables in this theory and shuffle them around in well-defined ways, we can construct out of them vectors in a different Hilbert space $mathcal H'$. That is, we have a linear map $T: mathcal H rightarrow mathcal H'$. This map is 1-to-1, meaning that we don't lose information. Strictly speaking, it doesn't need to be onto, but this distinction isn't crucial at first pass, so you can think of $T$ as a 1-to-1 correspondence (i.e. a linear isomorphism) if you like. In addition, the Hamiltonian $H$ can be mapped to $H'$, which describes compatible dynamics to $H$ on $mathcal H'$ (in the sense that one can first evolve in the original theory and then map to the new one, or first map and then evolve, and the results will be the same). When we look at the pair $mathcal H', H'$, we recognize this not as a $d+1$-dimensional theory of quantum gravity, but as a $d$-dimensional theory without gravity, but with extra symmetries that turn it into a so-called "conformal field theory" (which come from the extra symmetries of the AdS spacetime). In some sense, this new theory can be thought of as living on the boundary of AdS. So when we say that all the information is contained on the boundary, we mean that given a state $| psi' rangle in mathcal H'$ which describes the boundary theory observables, we can reconstruct the full state in the quantum gravity theory simply by $T^{-1}(|psi'rangle)$. What I've given here is just the 2-minute summary; AdS/CFT is still an active area of research and everything I said above is only approximately and/or morally true.

You might wonder why we need to rely on quantum mechanics. It turns out that (at least in the AdS/CFT correspondence), we can't do it classically. Highly quantum mechanical behavior in one theory is recovered by the classical limit of the other, and vice-versa. This is both a blessing and a curse, but at any rate there's no real classical equivalent. This is suspected to be true in any nontrivial example of the holographic principle, so we're pretty stuck describing things in terms of quantum information.

orbit - Can Pluto and Neptune collide anytime in future?

It's hard to find a good 2 dimensional representation, but Pluto and Neptune's orbits don't actually cross.

In fact, the orbits (not the planets but the orbits) never get within 100 million miles (I remember reading that, but can't find a link right now). This is because Pluto's orbit is on a different plane. It's easy to represent in 3D, hard to draw in 2D, but when Pluto is effectively the same distance from the sun as Neptune it's "below" Neptune, looking at the solar system from a sideways point of view. When it's on the same plane as Neptune's orbit, which it only crosses twice in one of it's years, it's further away from the sun.

Source_1 and Source_2 and Source_3

An astronomer diagrammed the 2 objects orbits for the next 4 billion years and they don't intersect during that time. Source

But there is a quirk in the calculations. Pluto's resonance with Neptune is probably long term unstable and because of this, it's orbit could be subject to variations that leave open a Jim Carrey's chance of hitting Neptune. Source

It's very unlikely, because they're not on the same plane and they're not orbiting at the same period and they're currently in orbital resonance which they would need to fall out of to even have a chance, so it's a lottery ticket's chance of happening but it could theoretically happen, in a billion or couple billion years.

It's more likely that Pluto would fly past Neptune and have it's orbit changed, and even that's still a long shot. The most likely scenario is that they never get close, at least over the lifespan of the sun, but if they do get close, Pluto could fly further out into the solar system or closer in due to gravity assist. It might even hit Earth. But, I repeat, this is crazy-long-shot territory, but I think it's worth mentioning that it's about as likely to hit one of the inner planets as it is to hit Neptune. Jupiter being the largest, might have the best chance of being hit by Pluto.

The question is also answered here, but because everyone else was answering this question no, I wanted to point out that it's just barely possible but extremely unlikely.

milky way - Galaxies are moving away from each other then how Milkyway and Andromeda galaxy coming towards each other?

Here is my answer to a similar question posted on the physics website.

Hubble's law (the law that deals with the expansion of the universe) applies to the expansion of space itself, i.e., if two objects stationary to each other that had no force between them were left alone the distance between would increase with time because space itself is expanding. This is what Hubble's law addresses.

In the case of the Milky Way and Andromeda galaxies (and all galaxies for that matter) there is a force between them: gravity. The gravitational force between the Milky Way and Andromeda galaxies has produced an acceleration that is causing the two galaxies to be moving towards each other faster than the space between them is expanding as calculated by Hubble's law. However, the vast majority of galaxies lie far enough away from the Milky Way that the gravitational force between us and them is small compared to the Hubble expansion and Hubble's law dominates.

In short, Hubble's law applies throughout the universe, but localized systems may have enough gravitational attraction between them that the gravitational effects dominate

How close are we to having the technology to measure planetary obliquity for exoplanets?

As far as I can tell, we do not yet have the precision to even put reasonable bounds on an exoplanets obliquity, but wikipedia seems to indicate that this may be possible in the "near future." It seems like this would have to be accomplished by direct imaging, either by directly observing rotational flattening of an exoplanets, or by looking for moons and assuming that the planet is tidally locked to the same plane as its satellite.

How close would you estimate we are to this kind of precision? Are there other approaches to measuring planetary obliquity?

Obviously, I'm not expecting a definite answer. Just wondering if anyone knows of any research in this area or has any thoughts about it.

date time - How could eclipse happen on the same day in a different calendar? (45 AD)

According to the eclipse calculator I use (Alcyone), a partial eclipse 0.314 magnitude occurred August 1, 45 A.D. in Rome starting at 8:35am local time.

This would seem to correspond to an eclipse which was predicted to fall on the birthday of the emperor Claudius in the consulship of Marcus Vinicius and Statilius Corvinus (45 AD), as described in Dio Cassius Book 6, Section 26. The birthday of Claudius was Kalends Augustus (the first of August).

If this is the case, it would seem that there is an exact correspondence between the Gregorian and Julian calendar going back to 45 AD. However, I would expect that small differences in leap years and other irregularities would make this exact match on the same day unlikely.

Another example is the eclipse of 1140 at London which the Saxon Chronicle says occurred on 13 Kalends April (March 20th) and Alcyone says occurred March 20, again a perfect match. Yet another example is the eclipse of 809 which passed over the Faroe islands and the Saxon Chronicle says occurred 17 Kalends Augustus (16 July) on the second day of the week. Now, of course, 16 July 809 is a Thursday in the Gregorian calendar, not the second day of the week, so something would seem to be fishy somewhere.

Is Alcyone/NASA making some kind of adjustment so that these pre-Gregorian dates match up?

dobsonian telescope - How could a hobbyist astronomer determine apparent magnitude of a star?

Apparent magnitude is a rather complex way to determine the brightness of a star. Quoting the introduction text from the linked to Wikipedia page:

The apparent magnitude (m) of a celestial body is a measure of its
brightness as seen by an observer on Earth, adjusted to the value it
would have in the absence of the atmosphere. The brighter the object
appears, the lower the value of its magnitude. Generally the visible
spectrum (vmag) is used as a basis for the apparent magnitude, but
other regions of the spectrum, such as the near-infrared J-band, are
also used. In the visible spectrum Sirius is the brightest star in the
night sky, whereas in the near-infrared J-band, Betelgeuse is the
brightest.

While it is of course a useful measure also for the slightly more casual, non-scientific observer to help determine the observed star from its neighboring cluster, or identification in general, I always wondered if there is a way to measure apparent magnitude with enthusiast-class equipment, and what would these procedures be?

                              

                              ^{Apparent magnitude scale and observational limits (Source: ESA Science)}

Also interesting would be a description of what level of precision and enthusiast can get with such equipment while measuring apparent magnitude of a distant star. If you need specifics to answer the question, such as precise available equipment or subject of observations, please feel free to choose at will any that would broadly match the capabilities of enthusiast-grade equipment.

navigation - What azimuth description systems are in use?

For a time, I thought Azimuth is always direction in degrees, 0 for local, geographic north, 90 for east etc.

Then, trying to observe moonrise I downloaded an app that gave azimuth in values between (if I recall correctly) -180 and 180, and I think "0" corresponded to south. I tried to read up on that and found very little fragmented information. I found other resources that frustrated me with numbers greater than 360, or provided values commonly associated with "West" for azimuth of moonrise and generally it took a bit of searching to find something with plain "degrees from north"

Could someone either post or provide a link to a comprehensive guide to various azimuth angle systems, when and where they are in use, and how to recognize which one is used?

Is the expansion of the universe proof of the big bang?

In and of itself, no the expansion of the universe is not proof of the big bang. Other theories could be constructed that would also be explained by the expansion. For example Fred Hoyle and others proposed the Steady State Theory which proposed that new matter was constantly being created thus causing the expansion. A the time there was no other evidence for either theory so either could have been true.

However, when you couple the expansion with other evidence such as the cosmic microwave background radiation discovered in 1964 then it does strengthen the case for the big bang being the cause.

atmosphere - Could spy satellites use laser guide stars (for adaptive optics)?

The guide star laser is used to adapt the optics at a specific time and location, for specific atmospheric conditions. It is not, as your question implies, used to calibrate the optics for long-term use.

Therefor, if order to calibrate the satellite with a laser fired from Earth, the reconnaissance target would have to provide the laser. This is useless for non-cooperative targets, which I would imagine make up the bulk of the reconnaissance targets.

I suppose that one could fire a laser from the satellite itself, but this would have the obvious disadvantage of making it even more obvious when a spy satellite was overhead.

In any case, the guide star image does not penetrate the entire atmosphere, I believe that it produces an image just a few tens of kilometers up. This is useful because it is the thicker, denser and more turbulent lower air of which is the most concern to optics. Firing the laser from above would thus see the laser energy absorbed in the thinner, stiller, more predictable upper atmosphere, thus it would have must less advantage.

One final reason not to use a guide star laser on a reconnaissance satellite would be power budget. These satellites are as small as possible, with as small a heat and reflectivity index as possible. The added power source, be it RTG, conventional batteries, or solar, would likely increase the reflectivity of the satellite.

cosmology - How exactly does inflation convert random gravity fluctuations into coherent gravitational waves?

In the course of this very enjoyable press announcement, it is mentioned that inflation can create gravity waves by amplifying gravity fluctuations.

I do not properly understand this statement. I always thought that quantum fluctuations (fluctuation of the metric in this case?) should occur pretty randomly whereas waves are rather coherent motions.

So I understand somehow that inflation can amplify quantum fluctuations, but I don't see how it can convert these random "microscopic" processes into "macroscopically" observable coherent phenomena called gravitational waves.
For example, how are the sources of these gravitational waves distributed? Does each point in space-time behave as some kind of a point source? And what is observed at a specific point as the superposition of all these excitations?

In addition, in the same video it is said that not all inflation models produce gravitational waves, the ones that have them are favoured by the data now, etc.