Apr 11, 2026 02:32 PM
(This post was last modified: Apr 11, 2026 03:37 PM by C C.)
The local universe’s expansion rate is clearer than ever, but still doesn’t add up
https://www.eurekalert.org/news-releases/1123728
EXCERPT: These two approaches are expected to yield the same result, but they don’t. Measurements based on the nearby Universe consistently indicate a higher expansion rate — around 73 kilometers per second per megaparsec — while predictions derived from the early Universe yield a lower value, closer to 67 or 68. Although the numerical difference is modest, it is far larger than can be explained by statistical uncertainty. This persistent disagreement, known as the Hubble tension, has now been observed across multiple independent studies and techniques... (MORE - missing details, no ads)
PAPER: http://dx.doi.org/10.1051/0004-6361/202557993
Cosmic inflation explains the universe’s low entropy at birth
https://bigthink.com/starts-with-a-bang/...hypothsis/
EXCERPTS: So how, then, did we — very ordered beings — emerge from this chaos? And if entropy has always been increasing, how did the Universe begin with an entropy that’s so much smaller than it is today? That’s the key behind a philosophical conundrum known as the past hypothesis puzzle. While philosophers may lose sleep over it, physicists don’t have to, because cosmic inflation solves it. Here’s how.
[...] here’s where the big question regarding the past hypothesis comes in: if each passing moment brings along with it an increase in entropy, and the entropy of the Universe has always been increasing, and the second law of thermodynamics dictates that entropy must always increase (or remain the same) and can never decrease, then how did our Universe initially start off in such a low-entropy state to begin with?
The answer, perhaps surprisingly, has been known theoretically for more than 40 years: cosmic inflation.
You might think of cosmic inflation in different terms, as either:
How does this occur?
The simplest way to explain it is to introduce two concepts to you that you likely have already heard of, but perhaps don’t have a sufficient appreciation of. The first is the difference between entropy (the total amount you’ll find) and entropy density (the total amount you’ll find in a given volume of space), which sounds easy enough. But the second requires a little bit of an explanation: the concept of adiabatic expansion. Adiabatic expansion is an important property in thermodynamics, in engines, in astrophysical gases and plasmas, and also in the expanding Universe...
[...] In other words, the solution to the problem of the past hypothesis puzzle, or why the Universe possessed a low-entropy state at the start of the hot Big Bang, is because the Universe underwent a period of cosmic inflation. The rapid, relentless, exponential expansion of the Universe took whatever entropy might have pre-existed within a specific region of space — occupying a certain volume of space — and inflated that volume to tremendous quantities.
Even though entropy was conserved (or possibly even increased very, very slightly), the entropy density plummets, as near-constant entropy in an exponentially expanding volume translates to having the entropy in any specific, well-defined volume of space becoming exponentially suppressed. That’s why, if you accept the evidence in favor of cosmic inflation, where that evidence is very, very strong, you no longer have a “past hypothesis” problem. The Universe is simply born with the amount of entropy that the transition from an inflationary state to a hot Big Bang state, a process known as cosmic reheating, imprints upon it.
The Universe was born in a low-entropy state because inflation caused the entropy density to plummet, and only after that did the hot Big Bang occur. Then, as expected, from that initial value, the Universe’s entropy has been forever increasing from that point onward. As long as you remember that entropy is not entropy density, and that inflating any volume while keeping the entropy constant necessarily results in a very small entropy density in the aftermath of that inflationary period, you’ll never be confused by the past hypothesis puzzle again... (MORE - missing details)
https://www.eurekalert.org/news-releases/1123728
EXCERPT: These two approaches are expected to yield the same result, but they don’t. Measurements based on the nearby Universe consistently indicate a higher expansion rate — around 73 kilometers per second per megaparsec — while predictions derived from the early Universe yield a lower value, closer to 67 or 68. Although the numerical difference is modest, it is far larger than can be explained by statistical uncertainty. This persistent disagreement, known as the Hubble tension, has now been observed across multiple independent studies and techniques... (MORE - missing details, no ads)
PAPER: http://dx.doi.org/10.1051/0004-6361/202557993
Cosmic inflation explains the universe’s low entropy at birth
https://bigthink.com/starts-with-a-bang/...hypothsis/
EXCERPTS: So how, then, did we — very ordered beings — emerge from this chaos? And if entropy has always been increasing, how did the Universe begin with an entropy that’s so much smaller than it is today? That’s the key behind a philosophical conundrum known as the past hypothesis puzzle. While philosophers may lose sleep over it, physicists don’t have to, because cosmic inflation solves it. Here’s how.
[...] here’s where the big question regarding the past hypothesis comes in: if each passing moment brings along with it an increase in entropy, and the entropy of the Universe has always been increasing, and the second law of thermodynamics dictates that entropy must always increase (or remain the same) and can never decrease, then how did our Universe initially start off in such a low-entropy state to begin with?
The answer, perhaps surprisingly, has been known theoretically for more than 40 years: cosmic inflation.
You might think of cosmic inflation in different terms, as either:
- the reason the Big Bang occurred,
- the additional, now-verified hypothesis of what came before and set up the conditions that the Big Bang was born with,
- or as the theory that removed the notion of the “Big Bang singularity” from the notion of the hot, dense, expanding state we identify as the Big Bang.
How does this occur?
The simplest way to explain it is to introduce two concepts to you that you likely have already heard of, but perhaps don’t have a sufficient appreciation of. The first is the difference between entropy (the total amount you’ll find) and entropy density (the total amount you’ll find in a given volume of space), which sounds easy enough. But the second requires a little bit of an explanation: the concept of adiabatic expansion. Adiabatic expansion is an important property in thermodynamics, in engines, in astrophysical gases and plasmas, and also in the expanding Universe...
[...] In other words, the solution to the problem of the past hypothesis puzzle, or why the Universe possessed a low-entropy state at the start of the hot Big Bang, is because the Universe underwent a period of cosmic inflation. The rapid, relentless, exponential expansion of the Universe took whatever entropy might have pre-existed within a specific region of space — occupying a certain volume of space — and inflated that volume to tremendous quantities.
Even though entropy was conserved (or possibly even increased very, very slightly), the entropy density plummets, as near-constant entropy in an exponentially expanding volume translates to having the entropy in any specific, well-defined volume of space becoming exponentially suppressed. That’s why, if you accept the evidence in favor of cosmic inflation, where that evidence is very, very strong, you no longer have a “past hypothesis” problem. The Universe is simply born with the amount of entropy that the transition from an inflationary state to a hot Big Bang state, a process known as cosmic reheating, imprints upon it.
The Universe was born in a low-entropy state because inflation caused the entropy density to plummet, and only after that did the hot Big Bang occur. Then, as expected, from that initial value, the Universe’s entropy has been forever increasing from that point onward. As long as you remember that entropy is not entropy density, and that inflating any volume while keeping the entropy constant necessarily results in a very small entropy density in the aftermath of that inflationary period, you’ll never be confused by the past hypothesis puzzle again... (MORE - missing details)