A lot of people have been asking me about the discovery announced this week about cosmic inflation. As I mentioned in my recent news post, you can find many good articles about it, and one of my favorites is this one from Phil Plait. Still, I realized that most people need a little more explanation of some basic concepts — such as of the ideas behind the “expanding universe” and the “Big Bang” — in order to understand the discovery and what it means. For that reason, I’ve put together the video below (or watch on Youtube) to try and provide the necessary context. My apologies for it being a bit rough, but hope you will find it useful.
Excellent video. Thank you.
The only part I didn’t get is the part about parts of the universe being so far away from each other, so that temperature exchange (to make it uniform) was impossible in the old model. (I do understand that inflation solves this problem).
How far are certain parts of the universe away from each other ? Or how far from each other are the most remote parts of the universe (is it correct to locate them at an angle of 180degrees from the observer’s point of view? 2 x 14 = 28 billion lightyears ?
Still looking for a website that explains this clearly.
Once again, thank you for your video.
Greetings from Belgium
Thanks for your comment. The details are more complex, but your basic idea is correct. Consider the cosmic microwave background in opposite directions in our sky. From each of those locations, the light was emitted when the universe was only 380,000 years old (the time at which the cosmic microwave background was released because the universe had cooled enough for photons could travel freely), which means each point had a cosmic horizon only 380,000 light-years in radius. But today they are 28 billion light-years apart, so there’s no way they could have had any mixing if expansion had proceeded uniformly (or close to uniformly) in a 14-billion-year-old universe. Inflation solves the problem because they were so much closer together than simple expansion would predict. A good analogy is the similar fossils found in eastern South America and western Africa. We are at first surprised because these places are separated by an ocean, but they help us understand that the two places were once in contact before the continents spread apart. The fact that the cosmic microwave background is the same temperature in all directions similarly tells us that these points were all once in close proximity, even though they are not today. Hope that helps!
>But today they are 28 billion light-years apart
Are they though? The photos have been traveling for about 14 billion years since recombination but the intervening space has also been expanding during that intervening time. I thought the distance between two opposing points on the surface of last scattering was more like 93 billion light years.
You are correct that “today” the distances are greater when we factor in the ongoing expansion, and that is where claims that the observable universe has a radius of 40+ billion light-years come from. However, any notion of “distance” in an expanding universe will always be ambiguous; e.g., do you mean an object’s distance at the time the light left, or now, or some time in between? The choice is arbitrary, with no particular choice (such as “now”) being any better than any other (such as “at the time the light left”). For that reason, my co-authors on The Cosmic Perspective and I have chosen to always equate “distance” with what is more formally called “lookback time” (the actual time it took the light to reach us). In other words, when we say a “distance” such as 14 billion light-years, we mean that the light took 14 billion years to reach us. The advantages of this approach are (1) there’s no ambiguity in the light-travel time; and (2) we can always infer lookback time directly from the given distance value, without needing any complex calculations of how expansion has affected distances since the light left. Interesting, most astronomers use the same convention that we do, but some physicists use the calculated distance instead. Personally, I think the latter approach just makes things more confusing for nonphysicists.
That makes more sense. I think the 93 billion light year distance is an attempt to answer the question while avoiding dealing with the ambiguity. Lookback time seems like a good stick to with which to beat up that avoidance.
Hi, I still do not quite get it.
Why in the Big-Bang standard model there is no sufficient time for equilibration, whereas there is sufficient time in the Inflation model?
Is it that in the standard model there is no 380,000 years period of inner radiation exchange before light could “escape” the grip of matter?
Does Inflation predict the 380,000 years of equilibration and then expansion?
The issue has more to do with distances than with time. If you run expansion backward without inflation, no matter how far back you go, points that are on opposite sides of our observable universe have always been farther apart than the light travel time between them. Today, for example, we can look 14 billion light-years in each direction, which means that points in opposite directions at those distances are 28 billion light-years away from each other — much too far for light to have traveled between them in a 14-billion-year-old universe. The only way around this problem is to hypothesize that there was a time when the universe expanded far faster than the current expansion rate implies. Inflation provides for this very fast expansion. To summarize: During the tiny fraction of a second before inflation occurred, every point in our observable universe (and far beyond) was close enough together so that it could come to an equilibrium temperature. Because all parts cool at the same rate based on the laws of physics, they’ve retained the same temperature even after inflation pushed these points apart.
Hey Patrik, I found this essay helpful:
http://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html