Questions tagged [structure-formation]

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Derivation of Press–Schechter mass function for $P(k)\propto k^n$

From 'Galaxy Formation and Evolution' by Mo, van den Bosch and White eq. (7.45) we have that $$ n(M,t)\ dM = \sqrt{\frac{2}{\pi}}\frac{\bar{\rho}}{M^2} \frac{\delta_c}{\sigma}\exp\left(-\frac{\delta_c^...
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What is the need for smoothing the cosmological density field?

I have recently started reading about the Press-Schechter formalism in cosmological structure formation, but what I still do not understand is why we need a smoothed density field in the first place. ...
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Does cosmology assumes that matter existed before the Big Bang?

In cosmology, studying the evolution of the matter perturbations for structure formation, one frequently mentions "horizon entry", meaning that a perturbation of (fixed) wavelength is super-...
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4answers
63 views

Why don’t almost all solid bodies merge into one body when they come in contact?

Macroscopic bodies in solid state when they touch each other do not connect in one monolithic body, because of electrostatic repulsion of their electron clouds. Okay. Say, have two iron ingot, we ...
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61 views

When the star formation begins?

We can separate the history of the universe in different epochs. Radiation dominated epoch, matter dominated epoch, and dark energy dominated epoch, and we can divide the epochs in different ways. ...
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Required Preconditions for Galactic Formation

In recent weeks, I have been working on building simulations to look at the formation of galaxies from dust clouds. It's a relatively crude simulation at this point as I needed a working model to ...
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1answer
63 views

Multiple levels of expansion of space? [closed]

It can easily be seen from the plot that there are at least 3 gaps between objects in the large, there is the gap between the planets and the moons, and between the planets and stars, and between ...
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1answer
56 views

Do post-big-bang quantum fluctuations determine star and planet formation?

It is my understanding that cosmology simulations largely implement general-relativity, taking quantum-effects into account merely by setting the fluctuations in the initial conditions. In the real ...
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1answer
51 views

Steinhardt order parameter and radial distribution function

Is there a relation between the probability distribution of Steinhardt order parameter for BCC and FCC and their radial distribution function? In the picture we see the probability distributions of ...
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1answer
107 views

Deriving the continuity equation for a perturbation from the continuity equation for the full density

I am attempting to derive the continuity equation for a density perturbation $\delta$, given the continuity equation for the full density $\rho(\mathbb{x}, t)$. This is in the context of cosmological ...
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Horizon entry, Meszaros suppression and start of perturbation growth

I thought that the onset of perturbation growth was determined by horizon entry of the perturbation (because there won't be a gravitational collapse of an over dense region not causally connected to ...
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1answer
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Which decoupled first? The dark matter or the CMB photons?

According theories that work best for the structure formation which happened first in the history of the evolution of the Universe, the decoupling of dark matter or the decoupling of photons? How sure ...
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1answer
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Decreasing water pressure toward top of tank walls

A contractor who is building a freestanding water storage tank of 6m (length) x 2m (width) x 1.5m (depth) suggests stepping out halfway up the walls (fibreglass sat in an iron frame) by 2cm to ...
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What is/causes this 3D dust structure?

I was recently on a flight and noticed some strange objects located inside the plane window. To me, it looks like dust which has arranged itself into a complex hydrocarbon-like structure, with ...
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Has the gaseous cosmic web ever been directly observed?

For a couple of decades now we've had maps of the cosmic web as traced by galaxy positions from redshift surveys like the 2dfgrs: Are there any similar maps which rely on gaseous tracers? I guess ...
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1answer
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What causes structure formation: Baryon acoustic oscillations or inflation?

Formation of structures such as galaxies and galaxy clusters require anisotropies in the cosmic plasma which (as I understand) were due to the anisotropies or density inhomogeneities created by Baryon ...
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55 views

Why do the sun and planets differ in their chemical elements composition?

The chemical elements composition of the sun changes as its nuclear furnace changes elements into heavier ones. But it is supposed to be mostly Hydrogen at the start. The planets are too cold for ...
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How are sand blocks formed?

My son asked me at the beach how are "sand stones" formed. These are the solid (but fragile) sand conglomerates which you can hold in your hand and which crumble to sand once pressed. I tried some ...
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Are galaxies merely groups of nearby stars that collected together, or do the stars share a common birth? [duplicate]

The news that some galaxies have dark matter and others do not leads to the question of how this could be possible. If galaxies are merely groups of stars that gathered together (under gravitational ...
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Gödel's Solution as background for a star patch

In Gödel's "Example of a New Type of Cosmological Solutions of Einstein's equation of Gravitation", all prior cosmological solutions with a non vanishing energy density have a property of all world ...
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Is there evidence the great attractor isn't just gravitationally lensed gravity?

Is there any evidence that the great attractor isn't just gravitationally lensed gravity? It seems odd that it would be directly on the other side of the black hole at the center of our galaxy and ...
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Why are all galaxies roughly the same size?

what determines the stable mass of galaxies, stars, star clusters or even galaxy clusters? Is there some obvious way to determine the classes of stable mass clusters from the initial conditions of the ...
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1answer
144 views

Can collapsing of the first star cause a staggering chain reaction throughout the universe?

Forgive me but I am not an academic but I have an enormous curiosity and am more or less teaching myself. I stumbled upon this site totally by accident and have relished the information here - ...
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146 views

Shell crossing in voids in spherical collapse model

What would be a necessary and sufficient for shell crossing to occur in an underdense region evolving based on spherical collapse model? i.e; shells moving outward with differential acceleration?
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If all the Structure in our visible Universe originated from Quantum Fluctuations, why isn't there as much Anti-Matter as Matter?

According to inflation, all the structure that we see like galaxies, etc. originated from quantum fluctuations which were expanded during inflation. But since quantum fluctuations always create ...
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1answer
92 views

Unique determination of displacement gradients in structural physics?

I am working on a 2-D planar problem in the x-y direction, dealing with stresses, strain, displacements. Under the linear elastic relation and after substitution I can write the following: $$ \begin{...
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What is the term for 'clean fracture vector'?

Today I observed a builder demolishing a wall in a house. He knew how to hit it in exactly the right place, with the right amount of force, for it to cleanly break into three smaller pieces - along ...
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1answer
236 views

In large scale cosmological structure, are filaments, walls, sheets and nodes different terms for the same things?

As the question says - are these all essentially synonyms referring to the same phenomenon (described with different words depending on the specific superficial appearance)? Or do they convey a real ...
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1answer
64 views

Newtonian derivation of perturbation in density

In Barbara Ryden's Introduction to Cosmology, chapter 12.3, she derives an equation describing the evolution of mass perturbations with time, for small perturbations $|\delta|\ll 1$. Before she starts ...
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Lower bound in the size of (naturally formed) black holes?

I suppose most of you will agree with me that Black Holes (BH) are among the coolest and most mysterious objects in our Universe. Theoretically predicted a century ago by the Einstein's theory of GR, ...
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What happens when we use the Virial Theorem iteratively?

Say I want to model the formation of structure in the Universe as a series of events whereby already virialised systems are brought together to create a larger virialised system. I will take the ...
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How is it possible to have such massive black holes? [duplicate]

Recent observations discovered really massive black holes, up to $20-40$ billions Solar masses. Now, according to an recent study and various computer simulations (I'm sorry, I don't have any ...
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1answer
95 views

What size of object does the peak of the cosmological power spectrum correspond to?

The title almost says it all, but to flesh it out more, what is the size a sphere corresponding to the peak in the cosmological power spectrum (Figure 2: https://ned.ipac.caltech.edu/level5/Sept11/...
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Structure formation: Galaxies as different entities and stars as groups

Why do stars exist in groups like galaxies rather than individual stars, and why do galaxies exist as different entities without continuity and with large intergalactic spaces?
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1answer
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What sets the lower mass limit for dark matter dominated structures?

Large galaxies appear to be dominated by dark matter, as do dwarf spheroidal galaxies with masses of perhaps $10^6 {\rm M}_{\odot}$. But (as far as I'm aware) there is no evidence for dark matter in ...
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148 views

Large scale structure of the universe

I have read of two seemingly straightforward explanations that give reasons for the inhomogeneity of the large-scale structure of the universe (i.e. why stars / galaxies formed where they did) One is ...
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64 views

Cosmology: Partical horizons and structure formation

In the concept of 'particle horizons' and structure formation. How would I show that before the matter-radiation equality, the proper particle horizon during this era evolves as Dh=2ct, where t is ...
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1answer
337 views

After being heated and cooled why does Coconut Oil form these structures?

According the the guy who posted this picture, the coconut oil melted during a heatwave and then re-solidified into hexagonal structures. I looked into foam physics and it seems that area deals with ...
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1answer
275 views

Is it always true that solid state matter is made up of repeating patterns?

In the chemistry book that I'm reading, it says: At even lower temperatures, the molecular movement becomes even more sluggish. The water molecules begin to align in a regularly repeating pattern,...
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1answer
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Question about the spiral formation

Im new to physics and i have been wondering about the spiral formation that can be seen occuring in Nature, e.g. Spiral galaxies, DNA strains, Bacteria strains, some flowers have the spiral form, also ...
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1answer
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Is graphene on a sticky tape strong enough to compete with my hand stretching it?

As we know we can make graphene using sticky tape so after we make graphene on a sticky tape will that thin sheet of graphene on a sticky tape be strong enough to compete with my hand trying to ...
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Validity of Press-Schechter formalism at the end of recombination

I was wondering if Press-Schechter formalism on Spherical collapse is applicable sometime at the end of recombination, i.e. the photons have started to free stream but the free streaming length is ...
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441 views

Structure formation: growth of density perturbations in sub-horizon vs super-horizon scales

I've always had difficulty visualizing how sub-horizon scales work versus super-horizon scales. Inflation causes perturbations due to quantum fluctuations. These are under-densities and over-...
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What makes Monocrystalline silicon a good semi-conductor? [closed]

I'm researching Monocrystalline silicon for a school project and focusing on its application in Electronics. One area I'm researching is why the material is a good semiconductor, and what properties ...
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1answer
83 views

Why Nitrides have internal polarization?

Why GaN has internal polarization? I know that in wurtzite crystal structure, the atomistic bonds are not equivalent and as a result there appears a net dipole and consequently a polarization. But ...
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Spatial dimension of galaxy filaments

Galaxy filaments are "amongst the largest known cosmic structures in the Universe. They [...] form the boundaries between large voids in the Universe." As their name suggests, filaments are ...
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1answer
322 views

Fate of largest scale structures?

In $\Lambda\mathrm{CDM}$, structures form "bottom up" with larger structures forming later. Structures are generally speaking supported by the velocity dispersion of their constituent objects (e.g. ...