Astronomy

Direct answer: Cosmologists classify the universe as open, flat, or closed based on spatial curvature. Current cosmic microwave background (CMB) measurements

In this article, we will look at why there are maximum mass limits for objects that are supported against gravity by degeneracy pressure instead of kinetic

In Part 1 of this 3-part series, I explained kinematics in Einstien-deSitter (EdS) cosmology and in Part 2, I explained kinematics in ##\Lambda##CDM cosmology

This is Part 2 of a 3-part series explaining evidence for so-called dark energy leading to a current positive cosmological acceleration. The evidence

Supernova (SN) Refsdal Key PointsAstronomers have been trying to understand space for thousands of years. Supernova Refsdal was the first known

In this 3-part series, I want to motivate the (re)introduction of the cosmological constant ##\Lambda## into Einstein's equations of general relativity

Big Bang: Key PointsHubble's law (redshift of galaxies) and the Doppler effect provide direct evidence that the universe is expanding away from our

Introduction The universe was not perfectly uniform at the beginning; some regions had higher density than others. Over time these higher-density regions

Overview Gravitational waves (GWs) are disturbances in spacetime produced by massive objects moving asymmetrically. Only the most massive and most relativistic

Uranus' Peculiar Tilt Uranus spins on its side. It has an obliquity (tilt) of 98 its rotation axis lies closer to the ecliptic plane than any other

Researchers simulating the accretion flow around Sagittarius A*, the supermassive black hole at the Milky Way's center, have found that magnetic reconnection

Introduction We are in many ways a fortunate generation having so many wonderful tools at our disposal on account of the "silicon revolution". The advent

This Insight is part of my attempt to develop a formal definition of 'large-scale isotropy', a concept that is fundamental to most cosmology, but that

A beginner telescope buyer should start with 7x50 binoculars and star charts, then join a local astronomy club to try different telescopes before buying.

Introduction This Insight explains how it is possible to calculate the spin of Sagittarius A* the supermassive black hole at the center of the Milky

Paper and context Paper discussion: Solar system expansion and strong equivalence principle as seen by the NASA MESSENGER mission. Antonio Genova, Erwan

The Big Bang was not an explosion from a point in space. It was the appearance of a singularity, a moment where spacetime itself breaks down and cannot

Previous Chapter:A Journey Into the Cosmos The Friedmann Equation Chapter 2-FLRW Metric and The Friedmann Equation In

In this Insight, we will discuss the general Robertson-Walker (RW) universe, in which a set of co-moving observers with proper time ##t## observe the universe

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

Introduction Some of the most visible features in our sky are solar system objects: the Sun, the Moon, and the planets. These objects are generally much

The 2017 August Eclipse really was something special. Fortunately Physics Forums has great photographers and snapped some remarkable shots. Here are a

 The CMB establishes a record of ancient acoustic oscillations in the baryon-photon plasma. We've been studying how these primordial sound

Series linksPart 1: Introduction to Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced AstrophotographyMotivation "Ive

Complete Series Part 1: Introduction Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced Astrophotography Level

Part 1: Introduction to Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced AstrophotographySeries & prerequisites For

 Before decoupling, photons and charged particles were in good thermal contact: though the primordial plasma might have varied in temperature

The Big Bang did not happen at a single point in space, so it has no center. Standard cosmological models based on general relativity treat the Big Bang

Are most visible stars brighter than the Sun? A couple of weeks back, a thread was started about whether or not most of the stars we see at night are

Big Bang and expansion (no inflation, no dark energy) Empty-universe solution (FLRW) Einstein's General Relativity allows a solution (the FLRW metric)

In Part I and Part II of this mini-series, we have briefly discussed the basic user interface and the use of charts to depict the LCDM cosmological model.

Neither of these questions makes sense in the form in which it was posed. To see why, let's start by thinking about how we know the universe is expanding.The

Background: textbook accounts Look in your standard cosmology textbook (in my case, the closest one at hand is Barbara Ryden's Introduction to Cosmology)

Part I dealt with the basic user interface of LightCone8. This part of the tutorial is about potentially useful cosmological insights to be gained from

Galaxy and quasar redshifts are classified as cosmological, not "intrinsic," because they match predictions from general relativity's expanding-universe

LightCone 8 is a versatile tabulating/charting cosmological calculator, useful for understanding the expansion history of the universe (and even some future

No. The second law of thermodynamics states that entropy can only increase over time. If the early universe had begun in a state of maximum entropy, the

The observable universe has a radius of about 46 billion light-years, far larger than its age of roughly 14 billion years. This happens because space itself

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.

The Big Bang was not a black hole because it happened everywhere in space at once rather than at a single point, and because the near-perfect homogeneity

All current astronomical observations are consistent with zero global rotation of the universe. Solar-system data set a model-independent upper limit of

 Astronomers Arno Penzias and Robert Wilson discovered the cosmic microwave background in 1965. They were not looking for it.They were

 The early universe was hot. So hot that nuclei boiled. The great thermal energy of the universe overwhelmed the confining efforts of the

The balloon analogy uses pennies glued to an inflating balloon to show two ideas from modern cosmology: that unbound regions of the universe expand roughly

 This is a picture of the cosmic microwave background:Fig 1.The cosmic microwave background as seen by the European Space Agency's

Introduction It is a common saying that during inflation "space expanded faster than the speed of light." This statement is meant to articulate the

Will all matter be converted into black holes and then photons? No. There is a misconception that a lot of laypeople seem to be picking up recently from

  Part 4: Cosmic Recession Rates An astronomer, accompanied by his amateur relativist friend, aimed a telescope at a distant galaxy and measured

  Part 3: Important Cosmological Horizons and Distances A question that often comes up is: "how big is the observable universe?"The question

  Part 2: Time and Distance In Part 1 we have seen how one of the simplest ways of expressing the fractional expansion rate of the universe,

 If we restrict ourselves to a spatially flat LCDM universe model (and ignore the early radiation energy density), the first Friedmann equation

How many days are there in a year? Ask pretty much anyone and they will tell you 365, except for leaps years which have 366. In addition,

A solar eclipse is safe to view with the naked eye only during totality, the brief period when the Moon completely covers the Sun. At every other stage

Stellar death outcomes depend on the star's initial mass: stars below roughly 8 solar masses end as white dwarfs, while more massive stars undergo core