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Big Bang Key Points Hubble‘s law (redshift of galaxies) and the Doppler effect provide direct evidence for the universe expanding away from our reference point, implying the universe is expanding. Low temperatures due to expansion from initial high temperatures at the Big Bang, supported by radio astronomy signals from stars, galaxies, radio galaxies, quasars, and…
In the cosmology section of the Physics Forums, I encountered many Big Bang and related infinity problems. Hence, in this article, I want to explain the Big Bang, the topology of the Universe, and the concepts of singularities and infinities. First, let’s start with the concept of the observable universe and what we mean by…
According to standard cosmological models, which are based on general relativity and are found to agree well with observations, time and space did not exist before the Big Bang — or even at the time of the Big Bang, which is a point where the theory breaks down because various quantities (such as temperature and…
The “ordinary Big Bang and expansion” (no inflation, no dark energy) Einstein’s General Relativity allows a solution (FLRW metric) where an empty Universe expands. Imagine you have particles (say, hydrogen atoms) in a cubic grid with exactly 1 light-year between the nearest particles. (We assume their mass is so tiny that this Universe essentially behaves…
In parts 1 and 2 of this 5-part Insights series, I explained the blockworld (BW) implication of special relativity (SR). Geroch sums up the BW perspective nicely with this quote[1]: There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as…
Standard cosmological models come in three flavors, open, flat, and closed,[Carroll] whose spatial curvatures are negative, zero, and positive. The open and flat types have infinite spatial volume. The closed one has finite spatial volume; spatially, it is the three-dimensional analog of the surface of a sphere. Since all three are solutions to the Einstein…
Abstract There is so much to say about the many endeavors by professional scientists to explain to us the world. The list is long: Carl Sagan, Harald Lesch, Neil deGrasse Tyson, Sabine Hossenfelder, Michio Kaku, and I even saw Roger Penrose and Steven Hawking on TV. The list is – of course – considerably longer…
Double-slit Key Points Quantum mechanics is known for its strangeness, including phenomena like wave-particle duality, which allows particles to behave like waves. The double-slit experiment is a key demonstration of this duality, showing that even single particles, like photons, exhibit wave-like behavior. When the experiment measures which slit a particle goes through, it behaves like…
In this 3-part series, I want to motivate the (re)introduction of the cosmological constant ##\Lambda## into Einstein’s equations of general relativity (GR) per the Supernova Cosmology Project (SCP) Union2.1 type Ia supernova data. As you probably know, this discovery won Perlmutter, Schmitt, and Riess the 2011 Nobel Prize in Physics “for the discovery of the accelerating…
People often claim on Physics Forums and in the foundations community proper that quantum mechanics is “incomplete.” Indeed, Lee Smolin recently wrote [1, p. xvii]: I hope to convince you that the conceptual problems and raging disagreements that have bedeviled quantum mechanics since its inception are unsolved and unsolvable, for the simple reason that the…
We are happy to have Michio Kaku answer some questions from the community. This interview was originally held in 2004. Michio Kaku is an American theoretical physicist, futurist, and popularizer of science (science communicator). He is a professor of theoretical physics in the City College of New York and CUNY Graduate Center. Kaku has written several books about physics and related topics, has made frequent…
In this Insight I will introduce the quantum mystery called “Wigner’s friend” using Healey’s version [1] of Frauchiger and Renner’s (FR’s) version [2] of Wigner’s version [3]. As with much of physics, the explication becomes more succinct and comprehensible with each successive rendering. I will show how this mystery results from treating classical information (behaving…
Clifford V. Johnson is a professor in the Physics and Astronomy Department at the USC. “I mainly work on (super)string theory, gravity, gauge theory and M-theory right now, which lead me to think about things like space-time, quantum mechanics, black holes, the big bang, extra dimensions, quarks, gluons, and so forth.” Clifford V. Johnson runs a…
This is an introduction to cosmology for someone who has some knowledge of calculus and basic physics. In this tutorial, we will take a journey into the cosmos to study cornerstone ideas in cosmology and their derivations. Let’s begin our journey, by explaining the meaning of cosmology. Cosmology is the study of the origin, evolution,…
The CMB establishes a record of ancient acoustic oscillations in the baryon-photon plasma. We’ve been studying how these primordial sound waves evolve, and how to analyze the last scattering surface to learn about them. Now it’s time to confront their origin: what process composed the cosmic symphony? A few different proposals have been advanced…
Before decoupling, photons and charged particles were in good thermal contact: though the primordial plasma might have varied in temperature from place to place, the photons and baryons were in local thermal equilibrium and shared a common temperature. If some physical process caused the baryons to heat up in some place, this temperature change…
We’ve been looking at how the continuum nature of spacetime poses problems for our favorite theories of physics — problems with infinities. Last time we saw a great example: general relativity predicts the existence of singularities, like black holes and the Big Bang. I explained exactly what these singularities really are. They’re not points…
Combining electromagnetism with relativity and quantum mechanics led to QED. Last time we saw the immense struggles with the continuum this caused. But combining gravity with relativity led Einstein to something equally remarkable: general relativity. In general relativity, infinities coming from the continuum nature of spacetime are deeply connected to its most dramatic successful…
Resistance to complex math seems to never die out. I see it frequently in PF posts. Often it takes the form of challenges rather than questions. First challenge: Complex is just a mathematical trick that has nothing to do with physics. Second challenge: Everything that complex does can be accomplished by ordinary real numbers. My…
How do we know that the redshifts of galaxies and quasars are cosmological and not “intrinsic?” Evidence for the cosmological interpretation The purpose of this FAQ entry is to explain why nonstandard interpretations of redshifts are not viable, not to explain from scratch how standard cosmological models were arrived at. The following is only a…
The radius of the observable universe is about 46 billion light-years, which is considerably greater than its age of about 14 billion years. The radius of the observable universe is defined by the greatest distance from which light would have had time to reach us since the Big Bang, so you might think that it…
Introduction to the Steady State Model Back in 2011, I wrote a FAQ entry for Physics Forums on the steady state model and why it is no longer viable. Since then, I’ve learned more about the theory, and it turns out that Fred Hoyle was a lot cleverer than I’d assumed. The following is an…
In the early universe, the matter was gathered together at very high density, so why wasn’t it a black hole? The first thing to understand is that the Big Bang was not an explosion that happened in one place in a preexisting, space. The Big Bang happened everywhere at once, so no location would be…
In parts 1 and 2 of this 5-part Insights series, I explained the blockworld (BW) implication of special relativity. In part 3, I introduced general relativity (GR) and brought BW to bear on general relativistic cosmology to ‘explain’ what puzzles so many people about big bang cosmology, i.e., the origin of the universe. The bottom…
In parts 1 and 2 of this 5-part Insights series, I explained the blockworld (BW) implication of special relativity (SR). In part 3, I introduced general relativity (GR) and brought BW to bear on general relativistic cosmology to ‘explain’ what puzzles so many people about big bang cosmology, i.e., the origin of the universe….
In part 1 of this 5-part Insights series, I introduced two consequences of the second postulate of special relativity (SR): time dilation (“moving clocks run slow”) and length contraction (“moving objects shrink”). Since moving clocks run slow, if you and observers at rest with respect to you (hereafter simply “you”) see me moving, you…
This is the first in a 5-part series of Insights that will introduce blockworld (aka “block universe”) and use it to address a puzzle (origin of the universe), paradoxes[1] (of closed timelike curves), and conundrums[2] (of quantum nonlocality) that I have seen discussed on Physics Forums. Blockworld (BW) says the past, present and future are equally ‘real’, i.e.,…
Astronomers Arno Penzias and Robert Wilson discovered the cosmic microwave background in 1965. They were not looking for it. They were using the comically distorted Holmdel Horn Antenna at Bell Labs, New Jersey, to study the reflection of radio waves off NASA balloon satellites. Despite all efforts to remove interference while calibrating the instrument…
The Balloon Analogy is a simple-minded way to help describe (but not completely describe) two facts of cosmology that are difficult for many people to see, namely that the universe is expanding uniformly and that there is no center (and no edge). The analogy is disliked (often intensely disliked) by serious physicists because it causes…
This is a picture of the cosmic microwave background: Fig 1. The cosmic microwave background as seen by the European Space Agency’s Planck satellite, 2013. If the universe is an 80-year-old man, here it is as a zygote only a tenth of a second old. It is the oldest light ever observed, and it tells…
Introduction It is a common saying that during inflation “space expanded faster than the speed of light.” This statement is meant to articulate the extreme rates of expansion seen during inflation, and this it does successfully. Unfortunately, it is at least a little wrong, and what little there is right about it is not unique…
It was hard to miss the 2011 OPERA neutrino speed measurement that indicated superluminal neutrino speeds (and turned out to be a measurement error), but measurements of neutrino masses and speeds have a long tradition. Neutrinos are very light particles that interact via the weak interaction and gravity only. There are three types of neutrinos:…
News of the LHC progress has dazzled scientists and hobbyists alike. It’s now time to show just how much you know about the operation. Please share your results Ready for your next quiz? How well do you know about the Periodic Table of Elements? 1. What is the Large Hadron Collider (LHC)? The Large Hadron…
Superdeterminism as a way to resolve the mystery of quantum entanglement is generally not taken seriously in the foundations community, as explained in this video by Sabine Hossenfelder (posted in Dec 2021). In her video, she argues that superdeterminism should be taken seriously, indeed it is what quantum mechanics (QM) is screaming for us to…
We often have questions about what telescope an aspiring amateur astronomer should buy. The “correct” answer can be elusive and is highly dependent on a lot of variables, including the expectations of the questioner, budget, storage capacity, available transport, etc. Rather than type all this advice over and over, I’d like to offer this post…
Previous Chapter: A Journey Into the Cosmos – The Friedmann Equation Chapter 2- FLRW Metric and The Friedmann Equation In this chapter, we will further investigate the Friedmann equation and we will explore the FLRW metric. We will solve the Friedmann equation for three different types of models (matter-dominated, radiation-dominated,…