“If you think this Universe is bad, you should see some of the others.” -Philip K. Dick
When it comes to measuring the expansion history of the Universe, the concept is simple enough: take something you know about an object, like a mass, a size, or a brightness, then measure what the mass, size or brightness appears to be, and suddenly, you know how far away that object has to be.
Add in a measurement of the object's redshift, and you can figure out not only what the expansion rate of the Universe is today, you can figure out the entire expansion history, and therefore what makes the Universe up. For practically all of the 20th century, we used brightness -- or standard candles -- exclusively. But new developments in both galactic surveys (like SDSS) and our understanding of dark matter and inflation has enabled us to use a new technique: baryon acoustic oscillations, or a standard ruler, for this task.
I suppose you'll soon do a blog on the latest ESO (Massey) result, claiming evidence that dark matter interacts with other dark matter?
Ethan, I didn't quite get something about how the BAO distance scale is actually measured. For the CMB, the full sky measurement corresponds to a relatively thin "surface" in time (the time of last scattering, which I gather is about 100,000 years thick), and so the angular distance we observe maps to a unique linear distance at Z~1100.
But for the SDSS data, the galaxies are spread out in three dimensions: RA and Dec on the sky, plus redshift/distance. From your figure, the redshift thickness corresponds to about half the age of the universe, and therefore to a huge range of different linear distances (stretched by the intervening expansion).
Is there enough data throughout to do the correlation measurement in slices of z? Or do you assume an expansion history (i.e., some a(t)) which allows you to integrate out the z dependence? Or am I missing something here that makes the angular correlation somehow independent of z?
There's some evidence of there being universal rotation. If this were true, what size of effect would be enough to explain the extra redshift of distant galaxies if it were not dark energy?
I haven't got the book to hand ("The 4% Universe") that mentioned it, with links to the papers, mind.
so they the nearest star to our solo-system is actually 4.3 light years away from our solo-system, and the special northern star Polaris is 232 light years away i understand this means it take light 232 years to reach the solo-system but those it mean the picture that we have of all the stars and galaxies is nothing but the past?
so they the nearest star to our solo-system is actually 4.3 light years away from our solo-system, and the special northern star Polaris is 232 light years away i understand this means it take light 232 years to reach the solo-system but those it mean the picture that we have of all the stars and galaxies is nothing but the past? i have been asking this question far a so long now i really need an answer
This is very interesting to learn about. I don't quite understand the concept of dark matter though. Would someone be able to tell me more about it or show me where i could find more information about it? It would be much appreciated. 15019838
> Is there enough data throughout to do the correlation measurement in slices of z?
Maybe I'm not understanding something here (likely in fact!). If I'm understanding correctly, upon observing two galaxies with a small angular separation, we can conclude that the distance to these galaxies is large. We assume that they are separated by the "standard ruler" distance and use that distance and some trigonometry to calculate how far away they are.
However, if two relatively close galaxies happen to lie along the same line of sight as seen from earth, would this not give misleading results. A small angular separation might also result from, for instance one galaxy at a distance of 300,000 ly and another at a distance of 800,000 ly, but both located such that they are on nearly the same line of sight. How would you distinguish between these two possibilities?
Certainly, for an extreme case such as I present, the redshift data could be used, but what about more distant galaxies? How would you distinguish two galaxies that are both about 2 billion ly away from one that's 2.5 billion ly away? Using redshifts would seem to me to be begging the question since it is precisely the relationship between redshift and distance that we are trying to investigate in that case.
Sorry, typo in my last paragraph. My question in that paragraph should be "how do you distinguish two galaxies that are about 2 billion ly away from one that is about 2 billion and one that is about 2.5 billion ly away, with the latter pair lying along nearly the same line of sight?"
It is amazing how science improves over time. I was just wondering whether there standard candle and standard ruler is about the same size or weather the one measurement unit is bigger than the other?
@Sean T #6: For relatively nearby galaxies, we have other standard candles we can use: Cepheid variables, type 1a supernovae, etc. Thus, we can get distance measures for some galaxies independently of redshift.
The perception theory is actually very helpful when it comes to knowing how far things are and their relative sizes but how accurate is this if we can't calculate the size of the galaxy, physically?
After one has read this, one starts to also think about how mass, size and brightness is perceived in different dimensions. Is it the same or different? Can someone inform me on that?
I know that all of the things we observe from a distance already happened and that we assume it as our present. Does this mean that if I observe from a further distance than that person that he already knows my future?
Thank you for the article Ethan.
If you are using size as a 'seems to be', you would probably have to do this visually. If this is done wont we be left with a useless result as we will be seeing the state of the object many hundreds of years in the past (depending on its distance from Earth)? Wouldn't Universal rotation also play a huge roll here, as the object under observation will currently be nowhere near its observed location?
Thanks once again.
wow the information presented in this blog is very intriguing considering the fact that the universe started out as a small dot and has expanded to the great and wonderful universe it is entailing galaxies and planet that are still unfamiliar to the human race. the ability to use the mentioned aspects to measure the expansion shows that we are not really that far from understanding the universe and being able to analyze how it operates and continue to gather more information. hopefully in the future we could really discover more about the other planets and actually find other forms of life in the planets. such research should be condoned and promoting in the current generation
The size of the universe is unfathomable. And what is even more compelling is whether the universe in question is finite or infinite. These are questions that scientists and astronomers have been battling with for ages. Furthermore, there are theories of the universe's end; Will it end the way it began, with a big bang due to too much expansion? Or a great shrink, the reversal of the origins of the universe? With breakthrough information, similar to that presented in the blog, we may get closer to answering these questions. Or could we be moving even further away from these answers?
Reading this blog I started thinking that, to my knowledge, there is no map of the Universe as it is "today". And I mean extrapolating the positions of celestial bodies, from planets to galaxies to place them in the position in the skies they really hold today. We can never see the present, only the past, but this way we could gain an impression of the current Universe.
When I look at the space-time cones and the vertex is at the Big Bang, I cannot help but think it should be the other way around: the observer is at the vertex and it's very little what he/she can observe of his/her present. As he/she looks away, he/she sees more and more things but farther out in the past. One never can see the Universe as it should be today.
Your thinking is quite correct. If you were able to travel to a star system, say, 1000 LY away, your path would not be dead straight, but curved, following the precession of that star over time.
For the two locations, each is mutually observing the past of the other. Even bringing this concept to a local scale, we observe each other in the past; minute as the distance might be between the two, it still takes time for light to travel the distance. To look at it slightly differently, when someone speaks to you from afar, you hear the words long after they leave the sender. In likewise, they hear your reply later, as well. So, it appears we all live in the present only where we personally are. Everyone, and everything else is in our past, whether femtoseconds, or LY apart.
Correction for the above comment. Should read @18, Rafael. Looks like another comment snuck in pre posting.
Referring to the expansion of the universe as they say: can we conclude that in the periodic table of the elements, the largest atom is the oldest?
Referring to the fact that the universe is expanding, as they say. Can we conclude that the largest atom on the periodic table of the elements is the oldest?
Wow the information presented in this blog is very intriguing, considering the fact that the universe started out as a small dot and has expanded to the great and wonderful universe it is entailing galaxies and planets that are still unfamiliar to the human race. The ability to use the mentioned aspects to measure the expansion shows that we are not really that far from understanding the universe and being able to analyze how it operates and continue to gather more information. Hopefully in the future we could really discover more about the other planets and actually find other forms of life in the planets, such research should be condoned and promoted in the current generation.
Observations that have been used such as red shift are extremely interesting to follow and analyze for research purposes. Dimensions and distances in the universe are truly mind blowing however hypothesis and theories may still be open for debate leaving astronomy an interesting subject to follow over the next few years.
This is very interesting to learn about. I don’t quite understand the concept of dark matter though. Would someone be able to tell me more about it or show me where i could find more information about it? It would be much appreciated.
Google it, Eiden
Wikipedia at the very least.
Measuring the universe's expansion accurately is a great step for science and due our capacities and understanding its not always that easy to grasp the distances and scales that we often talk about. Currently the diameter of the observable universe is +-93 billion lightyears, and expanding everyday. With this new method of determining the expansion rate we can most defiantly look forward to new discoveries.
Its amazing what we can discover with the tech available today. 10 years ago no one would of imagined that this could be possible.
Will the universe, ultimately expand until it all drifts apart?
@19, PJ, Stephan, It's even worse. Not even all of a person lives in its present, if "present" really has any meaning. Your hand's image, or even worse, sensory perception, is in your past when you register it. Even one neuron from another are in each other's past! There is really no present.
Those things DO exist, though, and they exist in time. So what do we call the time that they exist in?
Oh, the "present".
Seriously, trying the ideas of a 12-year-olds' idea of philosophy is completely asinine.