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/sci/ - Science & Math

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4147257 No.4147257 [Reply] [Original]

Can someone explain to me why the answer is C.

This is coming from the same assistant lecturer who said our galaxy is 2/3s from the center of the Universe.
>center of the universe

>> No.4147261

earth will collapse into the sun

>> No.4147287

>>4147261
LOL go on

>> No.4147295

i think the trick is "we will never be able to see it in real time."

only thing i can think of to make c the answer

>> No.4147306

it appears that the universe has a radius of 14B light years
also I think the consensus now is that the universe is expanding, so we'll never see anything outside of that edge
http://www.atlasoftheuniverse.com/universe.html

>> No.4147311

Do you seriously not get why this is true?
If the universe was created 15 billion years ago, then light from 15 billion years ago is the farthest out you can see.
Since nothing is faster than light, anything that was around from before then would be forever invisible to you.

>> No.4147315

ITT people who don't have neutrino telescopes

>> No.4147327

>>4147311
if you think about all the light emitted from the creation of universe coming from a single point it would make a sphere with a 30 billion light year diameter.

i.e. it's possible for two objects to be 16 billion light years away and still within the sphere.

>> No.4147334

>>4147327
No it isn't. Because the big bang happened everywhere.
So from where you are, something outside the radius would be invisible.
The universe isn't a big ball and you're on the inside. That's not how it works. Because if that were the case, you could be on one end of the ball and see 30 billion light years to the other end.

The universe is 15 billion light year radius from WHATEVER POINT YOU MEASURE from. Because it has no center and no shape.

>> No.4147338

>>4147334
interdasting, that i did not know.

>> No.4147350

>>4147334
If expansion is slower then the speed of light, wouldn't light from objects outside of that radius eventually become visible?

>> No.4147351

itt: people who think the universe is expanding at a 'rate' faster than the speed of light

>> No.4147367

>>4147334
you're mostly correct but on a technical note the universe is 45.7 billion light years in radius because of expansion and whatnot.

>> No.4147374

The light waves are still propagating through the expanding space. All light that isn't absorbed will make it to every part of the universe eventually. Time and distance are the SAME thing, in a relational sense.

>> No.4147380

>>4147350
yes, but the use of "We" in the question is vague.

>> No.4147393

>>4147367
so it would actually be possible to see the object, but because this isn't given in the question the answer remains as c?

>> No.4147401

>>4147380
This is how all the exam questions are written. And it's all only multiple choice...

>> No.4147402

>>4147334
But once, I went to the extremity of the universe. I left the auto-walk mode on and there was some invisible wall. I checked the data files and indeed the map ended there. How come?

>> No.4147409

>>4147380
Answer D alludes to the definition of "we".

It's just a poorly thought through question.

>> No.4147416

>>4147401
you have multiple choice EXAMS in college???

they really don't care about teaching anyone anything in america. I'm not going all /int/ i just can't believe they would sacrifice actually testing you on complicated (actual physics) problems so its easier to mark.

>> No.4147427

>>4147416
Well, it's an intro-like course at a public research university and it's a concept course, not numbers. But, I know. It's terrible.

>> No.4147442

the answer is D

>everything was created 15 billion years ago
Years is talking about time.

>object that is 16 billion light years away
Light years is talking about distance.

the distance light travels in 16b years.

therefore it would take 1 billion more years for that light to be able to reach you

[the two statements are both compatable. you could have a universe created 15 billion years (time) ago and have an object in that universe 1000 billion light years(distance) away. It would just take you 985 billion years to see that object that was created 15 billion years ago.]

>> No.4147445

This question is much too simple to be instructive.

It might be possible that the path of light would be distorted by a large solar mass, allowing us to observe, under the right conditions, objects 16b ly away...

>> No.4147464

>>4147445
gravitational lensing can do lots of things but it cannot make light get there faster.

>> No.4147474

>>4147442
Exactly. At least one person is on board with me.

I'm also still distraught over this guy using
>center of the universe

>> No.4147485

Unless there's a preface you're not showing us, it's an underspecified question, since it gives no information about how the Hubble rate changes over time.

>> No.4147489

>>4147464
I'm not thinking of getting there faster. If everything was created 16b years ago, everything is contained in a sphere of radius 16b light years.
That gives us 32b lights years of room to play with!

>Sling some light along a favourable geodesic and you're gravy baby.

>> No.4147509

>>4147485
No preface. This is how all the questions are.

And the Hubble constant could never really change...

>> No.4147520

>>4147489
>If everything was created 16b years ago
the question states 15 billion.

>> No.4147526

>>4147489
You are assuming that the universe started at a single point and expanded out at light speed. This would mean that Earth is at the center of the universe, since we can see around ourselves the same distance in all directions.

>> No.4147532

>>4147485
To elaborate on this, if the Hubble rate is constant, as in the case of a universe dominated by dark energy, C is the correct answer because the distance between us and any light emitted by the object expands at a rate faster than the light travels towards us. However, if the Hubble rate decreases, this result no longer follows. In an eternally expanding universe without dark energy, only having matter and radiation to make gravity, the sources of gravity die down, the Hubble rate approaches zero, and we get to see everything in the universe eventually, just like we would in flat spacetime. In a universe with a Big Crunch, there would be a limit to what we would be able to see. Our universe appears to be mostly dark energy, but it's not all dark energy, and the question probably isn't talking about our universe because of the not-quite-accurate number it uses for the age.

>>4147509
"Hubble constant" is a misnomer. It's not really a constant. It changes with time.

>> No.4147551

your in a theology class

>> No.4147563

>>4147532
But isn't 1 over the Hubble "constant", if you will, used to calculate the age of the universe? If the number changes, you'd be changing the age of the universe.

>> No.4147580

>>4147563
No. It works out approximately, but it isn't exactly right.

>> No.4147591

>>4147563
>>4147580
I should also add that Hubble rate = 1 / age of universe would be exact if there were no gravitation. And in this case, the Hubble rate would changing with time, since it would depend on the age of the universe.

>> No.4147606

>>4147563
>>4147580
>>4147591
And the reason 1/Hubble = age is a good approximation for our universe is because we had a period of time after the Big Bang when the universe was dominated by the ordinary gravity of matter, followed by a period dominated by the repulsive gravity of dark energy. They manage to approximately cancel each other out. It's only a numerological coincidence that it works as well as it does.

>> No.4147615

>>4147257
This is a stupid question because implies the two objects were created 15 billion light years apart. Which is complete BS - unless this is some completely theoretical universe model he's quizzing you on, in which case you need to post more of the question.

>> No.4147626

>>4147615
I'm not the OP, but I think it means 16 billion years away now. And OP told us that was all the info given. But I agree it's a horrible question, and given that his instructor is saying things like
>our galaxy is 2/3s from the center of the Universe
it sounds like the instructor doesn't really know anything about physical cosmology.

>> No.4147636

>>4147626
The instructor does his research in...goddamnit, I forget the name. But it's the designing of instruments and such to observe the universe.

>> No.4147638

>>4147257

>ITT: /sci/ doesn't know how to take multiple-choice tests

Maybe the question is oversimplified, but the answer in this context is obviously C. Answers A and B are obviously wrong, and answer D would presume a static universe. I'm sure your professor/ assistant lecturer has taught about expansion. As I understand it, we will never be able to see farther than the observable universe because expansion is accelerating, and expansion is not limited by the speed of light, but I could be wrong about that. But D is still wrong, because it would take more than 1 billion years. So just use process of elimination.

>> No.4147641

>>4147636
op, by the way

>> No.4147662
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4147662

>2011
>treating the universe as pointlike

>> No.4147688

>>4147638
>As I understand it, we will never be able to see farther than the observable universe because expansion is accelerating, and expansion is not limited by the speed of light, but I could be wrong about that.
You are wrong about that, but only in your conclusion "we will never be able to see farther than the observable universe." The observable universe is constantly increasing in size as light from more distant objects reaches us. But it won't ever include the whole universe, assuming dark energy continues to behave like a cosmological constant.

I agree that if you're given this test, you should answer C because you should be able to figure out that it's the answer the instructor wants. But that doesn't make this a good question. In a well-constructed question, you should be able to infer the right answer from reasonable assumptions about the problem. You shouldn't have to go with the only answer that could possibly be correct under any assumptions.