In this week's questions show, I explain why we'll never know which stars have no planets. How we could prevent a catastrophe to Earth, and why aliens might still be a threat to us.

Check out more of Dustin Gibson's photographs at:

https://www.instagram.com/gibsonpics/

00:48 How many stars don't have planets?

02:29 Could we prevent a catastrophe?

05:16 No aliens would be a threat to us

07:38 Can I have any name for Patreon?

08:20 Why not put a laser on the solar sail spacecraft

09:40 How can neutrinos go faster than light?

10:45 How do you calculate exposure times for Hubble?

14:32 What would cause stars to collide

17:19 Why don't we track asteroids?

18:55 How do magnetars form?

20:36 How could we destroy the planet?

22:45 Can a brown dwarf become a star?

Want to be part of the questions show? Ask a short question on any video on my channel. I gather a bunch up each week, and answer them here.

Our Book is out!

https://www.amazon.com/Universe-Today-Ultimate-Viewing-Cosmos/dp/1624145442/

Audio Podcast version:

ITunes: https://itunes.apple.com/us/podcast/universe-today-guide-to-space-audio/id794058155?mt=2

RSS: https://www.universetoday.com/audio

What Fraser's Watching Playlist:

https://www.youtube.com/playlist?list=PLbJ42wpShvmkjd428BcHcCEVWOjv7cJ1G

Weekly email newsletter:

https://www.universetoday.com/newsletter

Weekly Space Hangout:

https://www.youtube.com/channel/UC0-KklSGlCiJDwOPdR2EUcg/

Astronomy Cast:

https://www.youtube.com/channel/UCUHI67dh9jEO2rvK--MdCSg

Support us at: https://www.patreon.com/universetoday

More stories at: https://www.universetoday.com/

Twitch: https://twitch.tv/fcain

Follow us on Twitter: @universetoday

Like us on Facebook: https://www.facebook.com/universetoday

Instagram - https://instagram.com/universetoday

Team: Fraser Cain - @fcain / [email protected]

Karla Thompson - @karlaii / https://www.youtube.com/channel/UCEItkORQYd4Wf0TpgYI_1fw

Chad Weber - [email protected]

Support Universe Today podcasts with Fraser Cain

Very well made, it’s a big universe out there!

There is one error or bit of confusion which occurs in this presentation. It is that the cosmological event horizon is not the same as the CMB. The CMB is the domain where the opaque plasma stage of the universe ended, and is the surface of last scatter. The cosmological horizon is defined as a radius r = sqrt{3/Lambda}, where Lambda is the cosmological constant. Dark energy is the putative field effect which generates Lambda. The cosmological horizon is several billion light years out. Anything we observe with a z > 1 is beyond that limit.

The CMB occurs at a radius of about 45 billion light years. This is further away in distance than the age of the universe. What is happening is that the spatial surface of the universe evolves by sliding points away from each other, and for anything with z > 1 the frame of that object is being frame dragged by this physics at a velocity greater than light! for z not terribly large z ~= v/c. It is much the same reason a body which enters a black hole travels inwards at a velocity greater than light. The matter in the CMB region has a z ~= 1000. The relationship z ~ v/c no longer applies (one has to use general relativity) and so the CMB is moving away at only a few times the speed of light.

Of course it might sound paradoxical that something moving away faster than light can be seen by us with light. Yet a photon from such a source moving in our direction crosses regions expanding away from us at lower velocities. In this way the photon does eventually reach us. However, the cosmological event horizon prevents us from sending a return signal to anything with z > 1. This means that a mirror (if one big enough could be arranged for) at a great distance with z > 1 will not reflect back an image of our galaxy.

LC

To firm things up a bit, for z = 1 we are approximately at the cosmological horizon. The Hubble law v = Hd, for H = 68km/s/Mpc, can be used to get the distance d at v = c = 3e^5km/s. This is 4411Mpc, which is further than what I quoted above. This is one reason I did this, because on second though a “few billion light years” did not seem right. A parsec (pc) is 3.26 light years and so this is 14.3 billion light years out.

Now the exact relationship for z is

z = R(t)/R(0) – 1,

where R(0) is the initial radius of a region and R(t) is the radius now. For z sufficiently small the v/c relationship more or less works.

LC

The first part of this was truly magnificent, the best I’ve seen since the (unofficial, IIRC) SDSS-II presentation and its CMB comparison.

The second part was a bummer; while zooming out serves to embiggen the universe, zooming in belittle it. And nothing informative was added.

That doesn’t help, since as you said the observable CMB is at z ~ 10^3, and the cosmological horizon (not the event horizon) is what we see.

Also, the event horizon as I understand it is what we will ever be able to see – “in any event” :-o. Actually I believe z ~ oo would be a possible event horizon by relativity. And your relationship says so too, doesn’t it.

I don’t get the returning light discussion, as we only ever (have to) discuss one way communication in relativity?

I wonder what the Universe looks like in the present moment versus this chronological look back in time perspective?

Very nice graphic effects. Amazing how small everything gets just a few light years out.

The cosmological horizon is an event horizon. It is more similar to the Rindler horizon than the Kerr-Schwarzschild horizon of a black hole. It is inverted in a way, where we can see beyond it, just as an observer in a black hole can see out beyond the horizon once they have crossed it. So the one way communication is from the outside in to where we are, but we can’t communicate anything beyond the horizon.

Lawrence B. Crowell

I agree about the 13.7 billion years limit which is wrong. But in a it is very hard to represent such a thing in a video, especially when time has no direct meaning any more at such large scales.

The objects that we see at such a far distance has not the same time as we have right now.

Also what we see here at such a large distance is not what we would see at the furthest object. We would probably see the exact same thing around us with a horizon at 13.7 billion light years as we were here just different stars.

The years (13.7 billion years) is right. What is odd is that the universe out to the CMB sphere of last scatter is several times that in lightyears distance. Due to the frame dragging or commoving of particles on frame in an expanding space the distance is much larger. The CMB limit is 380,000 years after then big bang. Here the z ~ 1000, and if we can manage to peer beyond the CMB with neutrino astronomy or gravitational wave interferometers we could push z to enormous values, wich are z~ (R/L_{planck}) which is about 10^{50}! The universe is simply big — it could be infinite.

LC

Lawrence B. Crowell….information overload!

🙂

This reminded me of the beginning of the movie ‘Contact’, which did about the same thing, with not so much detail.

Seeing the ‘big picture’ tends to put perspective on most of the things we get angry or upset about. But I wax philosophical now….

Is there a High(er) resolution version available?

@ Lawrence B Crowell

Sir I do admire your science, but as you well

know, it would not be pausible to display this in such a short video? My question is, what is the purpose of your comment? Every UT.

reader knows, that it is a fictious representation of the fastness of our universe. We as homosapiens do not have the ability to observe nor comprehend this

soup which we call our universe. And for a

good reason! At best we are an neutrino on

and grain of sand traveling on a train. Looking

out the window we have the ability to sense

the motion by observing the movement of the surrounding objects! May I remind you that we

are moving at the speed of ligth as we are attached to this planet which is part of the

Milkyway! May I also remind you that the universe is an dynamic and primodial soup!

Nothing stand still and to say z=~1? I do call that an arbitrarely assumption? With all due

respect to all who trying to comprehend how it all connect, my simple brain has not been

able to make sense out of all the assumptions that has been throwen at me?

And you sir. have admitted on many occations, that we (the science community at large) do make arbitrarely assumptions?

This vedio violate the time line, and that is the

of what we perceived to be true? But it showed us how miniscule we are in respect

of the grand sheme we call the Universe!

May I whish you all a prosperous 2010?

The 45 Billion light year CMB radius “equation” seems irrelevant to me since we base everything on what can be seen in optical light measurements. Is that number based on the outward expansion of light in all directions of the Big Bang? We can’t see beyond the 13.73 Billion year light horizon, so I am having a hard time wrapping my brain around a number that is more than 4 times the age of the Universe, which has been accepted for so many decades.

Also….

I recently read that it had been determined that the Universe was flat (which I have a really hard time with). BUT, If it is flat, doesn’t that change a boat load of calculations and perceptions of how we see and understand our Universe? Wouldn’t that change the trajectory of the light we capture? Pardon my ignorance, but these questions I had to ask….

A course in general relativity is probably advised in order to understand how it is that the CMB can that far out. The reason is similar to why anything in a black hole can’t escape because spacetime is dragging it inwards faster than light. The speed of light is an invariant measured in any local region. Space or spacetime on the other hand across some distance can morph or do otherwise.

LC

Just shut-up and enjoy the work done. Nobody cares about how much crap you can search for on Google, and thern cut-and-paste on this site.

You aren’t showing intelligence… you are displaying pig-headedness.

I am not copying this from websites: I actually know and work with these concepts. I went to a planetarium presentation last year where the presentation got this wrong. It might sound like nitpicking, but in reality this is a big issue, and the universe is a whole lot bigger as a result.

LC

@Aodhhan:

A bit of jealousy, perhaps? Or, did some bully steal your first telescope when you where in grammar school? Just learn, don’t hate!

People who know things and work hard at it or anything… recognize someone elses work when a lot of time is put into it. This happens to be one of it.

Prima-donnas on the other hand, have no idea or appreciation about work.. so they go around nit-picking or adding rediculous information which is usually out of place.

..and the most obvious… they know something about everything.

Lawrence… you have no formal education in astronomy. Likely none in any of the sciences. This is evident by your many postings. Your information and the way you present it isn’t consistent… in fact at times… the correct words are used… they are just used improperly….often times in a way which is rather (laugh at YOU) funny

I think this is more a case where you don’t understand this. It is a good this does not involve quantum black hole holography — that is really strange. The universe beyond the comsological horizon is being frame dragged outwards faster than our local speed of light. You should look at Ned Wright’s “Cosmology Tutorial” website to read up more on this.

As for formal education, I am not an astronomer, but I did doctoral work on gravitation. Please keep your vituperations down, particularly when it involves things which sound paradoxical because you don’t understand them.

LC

I have subscribed to this site for several years now specifically to learn as much as I can about our place in space and all the cool stuff that got us here to this tiny little spot we call home.

I always respect the comments I see here – factual or funny – whether I agree or not and have asked questions when I didn’t understand something.

It would be seriously disappointing to find out that even here can be just as trivial and petty as all the other crap websites out there that really have no reason to exist. Sites like this one matter because they educate and inform.

Let’s continue stand taller than that, please.

(I especially appreciate the fact that there is NEVER a video of some A*hole getting hit in the crotch!)

Which reminds me –

Has anyone else out there read that the Universe was recently determined to be FLAT???

I can’t seem to find any articles on that particular subject so I am obviously looking in the wrong places.

Any ideas???

The Friedmann-Lemaitre-RobertsonWalker (FLRW) metric contains a discrete parameter k = {-1, 0, 1}. For k = 1 the spatial surface is a three dimensional sphere. In this case the spacetime consists of a foliation of three dimensional spheres S^3 which expand and depending on the value of the cosmological constant will continue to do so or recollapse. For k = 0 the spatial surface is a flat space R^3, and these foliate out a spacetime. Each point on a particular R^3 is connected to other points on a preceding R^3 by a time “arrow” t^a = N^a + Ne^a. N is the so called lapse function which tells how far to move up to the next R^3 surface, and N^a is a shift function which tells each point how to slide relative to other points. This holds for the k = 1 case as well, but is more interesting in this case. So you have points on a spatial surface which are pushed forwards to the next surface and then slide along some spatial direction on that next surface. In the case of the observable universe all points slide away from each other, which is the expanding universe, and further this expansion accelerates according to a cosmological constant /\ — called Lambda. The k = -1 case is for a universe with a negative curvature, sometimes referred to as the saddle shaped universe. This space and its ensuing spacetime has certain strange properties which make it unlikely. For k = 0 the space might be flat, but this evolution of sliding points to successive spatial surfaces means these flat spaces are embedded in a spacetime with curvature. Here the curvature is associated with the time direction.

The FLRW equations of motions describe the evolution of a region of space with a radius R. If we assume that k = 0 then this differential equation of motion assumes a particularly simple form

R” = c^2/\R/3. ‘ = time derivative d/dt, “ = d^2/dt^2

The cosmological horizon, something which my mention of here cause some umbrage, occurs at a distance r = sqrt(3//\}. So if we assume that the region of dynamic evolution we are considering is the region contained in this volume then /\/3 = 1/R^2. This results in the simple equation R” = c^2/R, which is the same equation for the “gravity” involved with the production of Hawking-Unruh radiation for a stationary observer a distance R from the event horizon. The other spacetimes with k = 1 or -1 deviate from this, and the gravity associated with the expansion of the universe is not commensurate with principles of quantum fields in curved spacetime. It is also the condition which permits quantum holography of quantum fields or strings interacting with black holes. This matter is a very strange topic, and to understand it you need to suspend certain ideas about sequences of events and the uniqueness of trajectories. I am not going to delve into this as any more length, for you have to do as Louis Carroll’s Red Queen advises and think of several impossible things at the same time.

The data does also suggest that the spatial surface of the universe is flat as well. The COBE and WMAP data on the distant CMB indicates no spatial curvature across that vast distance. Further, the data indicate that the cosmological constant has a particular structure according to a vacuum energy density and its pressure. This condition is where the pressure is equal to the negative of the density (with factors of c) .

There is another strange issue, for if spacetime has a constant energy density, called dark energy, then the expansion of the universe should be effectively creating energy by its expansion. The metric has time dependent metric elements, which means there is no Killing isometry which defines energy conservation as a symmetry of the spacetime. Now again this might punch some people’s conceptions of things, but cosmological spacetimes are such that energy can be generated globally and there is no global conservation law of energy. Yet for the flat R^3 while regions might expand the global space does not, and this (with some care I will not discuss here) avoids the problem of creating energy out of nothing.

In the end there are strong theoretical reasons to think k = 0 or space is flat, and the data strongly indicates this is the case as well.

LC