On the role and the implications of large-scale peculiar motions on the deceleration parameter of the universe

2021 ◽  
Author(s):  
Μιλτιάδης Καδιλτζόγλου
Keyword(s):  
Deceleration Parameter ◽  
Large Scale ◽  
Milky Way ◽  
Bulk Flow ◽  
The Universe

Κανένας ρεαλιστικός παρατηρητής δεν ακολουθεί την ομαλή διαστολή του σύμπαντος, την λεγόμενη «ροή Hubble», αλλά όλοι κινούμαστε ως προς αυτή. Ειδικότερα, ο Γαλαξίας μας και το τοπικό σμήνος γαλαξιών, κινούνται με ταχύτητα περίπου 600km/s. Επίσης, ένας μεγάλος αριθμός πρόσφατων ερευνών έχει επανειλημμένα επιβεβαιώσει την παρουσία ίδιων κινήσεων μεγάλης κλίμακας, τις επονομαζόμενες και «bulk flows». Παρόλα αυτά, οι ίδιες κινήσεις συνήθως παρακάμπτονται στις περισσότερες θεωρητικές κοσμολογικές μελέτες, ενώ στις ελάχιστες που συνυπολογίζονται η ανάλυση είναι σχεδόν αποκλειστικά Νευτώνεια. Επί πλέον, οι μελέτες γίνονται από τη σκοπιά του ιδεατού παρατηρητή, αυτού που ακολουθεί την ομαλή διαστολή Hubble, και όχι του πραγματικού που έχει μία σχετική ιδιοταχύτητα. Αυτό έχει ως αποτέλεσμα οι επιπτώσεις της ιδίας κίνησής του γαλαξία μας, ως προς την ομαλή διαστολή του σύμπαντος, να μην συνυπολογίζονται. Ωστόσο, είναι από καιρό γνωστό ότι φαινόμενα που συνδέονται με σχετική κίνηση επηρεάζουν τον τρόπο με τον οποίο οι παρατηρητές ερμηνεύουν το περιβάλλον τους. Μάλιστα, η ιστορία της αστρονομίας είναι γεμάτη παραδείγματα όπου η σχετική κίνηση έχει οδηγήσει σε καταφανή παρερμηνεία της πραγματικότητας. Ο σκοπός της συγκεκριμένης διατριβής είναι να προσφέρει μία σχετικιστική μελέτη των ίδιων κινήσεων και να ερευνήσει τις επιπτώσεις τους στον τρόπο με τον οποίο αντιλαμβανόμαστε τη μέση κινηματική συμπεριφορά του σύμπαντος που μας περιβάλει και πιο συγκεκριμένα τον ρυθμό επιτάχυνσης/επιβράδυνσης αυτού. Ο τελευταίος προσδιορίζεται από την παράμετρο επιβράδυνσης, η οποία παραδοσιακά είναι θετική όταν το σύμπαν επιβραδύνεται και αρνητική όταν επιταχύνεται. Εισάγοντας ένα «κεκλιμένο» (tilted) κοσμολογικό μοντέλο, επιτρέπουμε δύο οικογένειες παρατηρητών. Η πρώτη ακολουθεί την ομαλή ροή Hubble, η οποία ορίζει και το σύστημα αναφοράς του σύμπαντος, ενώ η δεύτερη οικογένεια ζει σε έναν τυπικό γαλαξία, όπως ο δικός μας (Milky Way), και κινείται ως προς την πρώτη. Θεωρώντας ότι το σύμπαν περιγράφεται από ένα διαταραγμένο κοσμολογικό μοντέλο Friedmann και υποθέτοντας ύλη με μηδενική πίεση, την επονομαζόμενη «σκόνη», δείξαμε ότι η παράμετρος επιβράδυνσης που μετρούν οι δύο παραπάνω παρατηρητές μπορεί να παίρνει αισθητά διαφορετική τιμή στα συστήματα αναφοράς τους και ότι η διαφορά αυτή οφείλεται αποκλειστικά και μόνο στη σχετική τους κίνηση. Επιπλέον, κάνοντας χρήση σχετικιστικής θεωρίας γραμμικών κοσμολογικών διαταραχών, δείξαμε ότι παρατηρητές που βρίσκονται εντός μίας ελαφρά συστελλόμενης bulk flow ενδέχεται να προσδίδουν αρνητικές τιμές στην δική τους, την τοπικά μετρούμενη, παράμετρο επιβράδυνσης, ενώ το σύμπαν σαν σύνολο να επιβραδύνεται. Αν και το γεγονός αυτό είναι τοπικό και οφείλεται αποκλειστικά στην ιδία κίνηση του παρατηρητή, οι περιοχές που επηρεάζονται είναι συνήθως αρκετά μεγάλες (από μερικές εκατοντάδες Mpc έως αρκετές εκατοντάδες Mpc), ώστε να δημιουργείται η λανθασμένη εντύπωση ότι ολόκληρο το σύμπαν έχει πρόσφατα περάσει σε φάση επιταχυνόμενης διαστολής. Ενδείξεις για την ορθότητα του παραπάνω σεναρίου και της πιθανότητας η πρόσφατη επιταχυνόμενη διαστολή του σύμπαντος να αποτελεί μία ψευδαίσθηση και ένα δημιούργημα της ιδίας κίνησης του γαλαξία μας, θα πρέπει να αναζητηθούν στα παρατηρησιακά δεδομένα. Αυτά, εκτός των άλλων, πρέπει να εμπεριέχουν την χαρακτηριστική υπογραφή, το «σήμα κατατεθέν», των ιδίων κινήσεων, δηλαδή μία φαινομενική (τύπου Doppler) διπολική ανισοτροπία που θα οφείλεται στην κίνηση του παρατηρητή. Με άλλα λόγια, στα παρατηρησιακά δεδομένα, το σύμπαν θα πρέπει να φαίνεται ότι επιταχύνεται ταχύτερα προς μία κατεύθυνση της ουράνιας σφαίρας και εξίσου βραδύτερα προς την αντιδιαμετρική. Τα τελευταία δέκα χρόνια υπάρχουν αρκετές αναφορές στη βιβλιογραφία ότι ένας διπολικός άξονας, όπως αυτός που προαναφέρθηκε, μπορεί πράγματι να υπάρχει στα δεδομένα των σουπερνόβα. Με άλλα λόγια, το σύμπαν μας μπορεί πράγματι να φαίνεται πως επιταχύνεται πιο γρήγορα προς μία κατεύθυνση στον ουρανό και εξίσου πιο αργά κατά μήκος του αντίποδα.

scholarly journals A kinematic confirmation of the hidden Vela supercluster

2019 ◽  
Vol 490 (1) ◽  
pp. L57-L61 ◽  
Author(s):  
Hélène M Courtois ◽  
Renée C Kraan-Korteweg ◽  
Alexandra Dupuy ◽  
Romain Graziani ◽  
Noam I Libeskind
Keyword(s):  
Excellent Agreement ◽  
Mass Concentration ◽  
Large Scale ◽  
Milky Way ◽  
Large Scale Structure ◽  
Peculiar Velocity ◽  
The Galaxy ◽  
Zone Of Avoidance ◽  
First Time ◽  
The Universe

ABSTRACT The Universe region obscured by the Milky Way is very large and only future blind large H i redshift, and targeted peculiar surveys on the outer borders will determine how much mass is hidden there. Meanwhile, we apply for the first time two independent techniques to the galaxy peculiar velocity catalogue CosmicFlows−3 in order to explore for the kinematic signature of a specific large-scale structure hidden behind this zone: the Vela supercluster at cz ∼18 000 km s−1. Using the gravitational velocity and density contrast fields, we find excellent agreement when comparing our results to the Vela object as traced in redshift space. The article provides the first kinematic evidence of a major mass concentration (knot of the Cosmic Web) located in the direction behind Vela constellation, pin pointing that the Zone of Avoidance should be surveyed in detail in the future.

scholarly journals The peculiar Jeans length

2021 ◽  
Vol 81 (8) ◽  
Author(s):  
Christos G. Tsagas
Keyword(s):  
Relative Motion ◽  
Deceleration Parameter ◽  
Bulk Flow ◽  
The Other ◽  
Characteristic Scale ◽  
Hubble Expansion ◽  
Other Hand ◽  
Peculiar Motion ◽  
The Universe

AbstractTypical observers in the universe do not follow the smooth Hubble expansion, but move relative to it. Such bulk peculiar motions introduce a characteristic scale that is closely analogous to the familiar Jeans length. This “peculiar Jeans length” marks the threshold below which relative-motion effects dominate the linear kinematics. There, cosmological measurements can vary considerably between the bulk-flow frame and that of the Hubble expansion, entirely due to the observers’ relative motion. When dealing with the deceleration parameter, we find that the peculiar Jeans length varies between few and several hundred Mpc. On these scales, the deceleration parameter measured by the bulk-flow observers can be considerably larger (or smaller) than its Hubble-frame counterpart. This depends on whether the peculiar motion is locally expanding (or contracting), relative to the background expansion. Then, provided expanding and contracting bulk flows are randomly distributed, nearly half of the observers in the universe could be misled to think that their cosmos is over-decelerated. The rest of them, on the other hand, may come to believe that their universe is under-decelerated, or even accelerated in some cases. We make two phenomenological predictions that could in principle support this scenario.

scholarly journals On the Mass Assembly History of the Local Group

2019 ◽  
Author(s):  
Edoardo Carlesi ◽  
Yehuda Hoffman ◽  
Stefan Gottlöber ◽  
Noam I Libeskind ◽  
Alexander Knebe ◽  
...  
Keyword(s):  
Large Scale ◽  
Cold Dark Matter ◽  
Milky Way ◽  
Local Group ◽  
Cosmological Parameters ◽  
Major Merger ◽  
History Of ◽  
Mass Assembly ◽  
Theoretical Considerations ◽  
The Universe

Abstract In this work an ensemble of simulated Local Group analogues is used to constrain the properties of the mass assembly history of the Milky Way (MW) and Andromeda (M31) galaxies. These objects have been obtained using the constrained simulation technique, which ensures that simulated LGs live within a large scale environment akin to the observed one. Our results are compared against a standard Λ Cold Dark Matter (ΛCDM) series of simulations which use the same cosmological parameters. This allows us to single out the effects of the constraints on the results. We find that (a) the median constrained merging histories for M31 and MW live above the standard ones at the 1-σ level, (b) the median formation time takes place ≈0.5 Gyr earlier than unconstrained values, while the latest major merger happens on average 1.5 Gyr earlier and (c) the probability for both LG haloes to have experienced their last major merger in the first half of the history of the Universe is $\approx 50\%$ higher for the constrained pairs. These results have been estimated to be significant at the $99\%$ confidence level by means of a Kolmogorov-Simirnov test. These results suggest that the particular environment in which the Milky Way and Andromeda form plays a role in shaping their properties, and favours earlier formation and last major merger time values in agreement with other observational and theoretical considerations.

Cosmological spacetimes

2018 ◽  
Author(s):  
Nathalie Deruelle ◽  
Jean-Philippe Uzan
Keyword(s):  
Cosmological Model ◽  
Large Scale ◽  
Cosmic Time ◽  
De Sitter ◽  
Matter Distribution ◽  
Observable Universe ◽  
Cosmological Principle ◽  
Riemannian Spacetime ◽  
The Universe ◽  
Copernican Principle

This chapter provides a few examples of representations of the universe on a large scale—a first step in constructing a cosmological model. It first discusses the Copernican principle, which is an approximation/hypothesis about the matter distribution in the observable universe. The chapter then turns to the cosmological principle—a hypothesis about the geometry of the Riemannian spacetime representing the universe, which is assumed to be foliated by 3-spaces labeled by a cosmic time t which are homogeneous and isotropic, that is, ‘maximally symmetric’. After a discussion on maximally symmetric space, this chapter considers spacetimes with homogenous and isotropic sections. Finally, this chapter discusses Milne and de Sitter spacetimes.

The Long View of Planetary Systems

2018 ◽  
Author(s):  
Karel Schrijver
Keyword(s):  
Solar System ◽  
Milky Way ◽  
Planetary Systems ◽  
Giant Planets ◽  
Milky Way Galaxy ◽  
Population Statistics ◽  
The Past ◽  
General Terms ◽  
The Galaxy ◽  
The Universe

How many planetary systems formed before our’s did, and how many will form after? How old is the average exoplanet in the Galaxy? When did the earliest planets start forming? How different are the ages of terrestrial and giant planets? And, ultimately, what will the fate be of our Solar System, of the Milky Way Galaxy, and of the Universe around us? We cannot know the fate of individual exoplanets with great certainty, but based on population statistics this chapter sketches the past, present, and future of exoworlds and of our Earth in general terms.

The Nature of Time

2018 ◽  
Author(s):  
Donald C. Williams
Keyword(s):  
Large Scale ◽  
Time Travel ◽  
Dimensional Manifold ◽  
Nature Of Time ◽  
Manifold Theory ◽  
The Universe ◽  
Theoretical Cost ◽  
Do So

This chapter provides a fuller treatment of the pure manifold theory with an expanded discussion of competing doctrines. It is argued that competing doctrines fail to account for the extensive and/or transitory aspect(s) of time, or they do so at great theoretical cost. The pure manifold theory accounts for the extensive aspect of time because it admits a four-dimensional manifold and it accounts for the transitory aspect of time because it hypothesizes that the increase of entropy is the thing that is ‘felt’ in veridical cases of felt passage. A four-dimensionalist theory of time travel is outlined, along with a sketch of large-scale cosmological traits of the universe.

scholarly journals Large-Scale Flows in the Local Universe

1996 ◽  
Vol 168 ◽  
pp. 175-182 ◽  
Author(s):  
D.S. Mathewson ◽  
V.L. Ford
Keyword(s):  
Large Scale ◽  
Velocity Measurements ◽  
Local Universe ◽  
Peculiar Velocities ◽  
Redshift Surveys ◽  
Density Maps ◽  
The Universe ◽  
Great Wall ◽  
Great Attractor ◽  
Streaming Flow

Peculiar velocity measurements of 2500 southern spiral galaxies show large-scale flows in the direction of the Hydra-Centaurus clusters which fully participate in the flow themselves. The flow is not uniform over this region and seems to be associated with the denser regions which participate in the flow of amplitude about 400km/s. In the less dense regions the flow is small or non-existent. This makes the flow quite asymmetric and inconsistent with that expected from large-scale, parallel streaming flow that includes all galaxies out to 6000km/s as previously thought. The flow cannot be modelled by a Great Attractor at 4300km/s or the Centaurus clusters at 3500km/s. Indeed, from the density maps derived from the redshift surveys of “optical” and IRAS galaxies, it is difficult to see how the mass concentrations can be responsible particularly as they themselves participate in the flow. These results bring into question the generally accepted reason for the peculiar velocities of galaxies that they arise solely as a consequence of infall into the dense regions of the universe. To the N. of the Great Attractor region, the flow increases and shows no sign of diminishing out to the redshift limit of 8000km/s in this direction. We may have detected flow in the nearest section of the Great Wall.

scholarly journals Investigating the properties of a galaxy group at z = 0.6

2020 ◽  
Vol 15 (S359) ◽  
pp. 188-189
Author(s):  
Daniela Hiromi Okido ◽  
Cristina Furlanetto ◽  
Marina Trevisan ◽  
Mônica Tergolina
Keyword(s):  
Large Scale ◽  
The Other ◽  
Numerical Density ◽  
Spectroscopy Data ◽  
Stellar Kinematics ◽  
Galaxy Groups ◽  
The Galaxy ◽  
The Difference ◽  
The Impact ◽  
The Universe

AbstractGalaxy groups offer an important perspective on how the large-scale structure of the Universe has formed and evolved, being great laboratories to study the impact of the environment on the evolution of galaxies. We aim to investigate the properties of a galaxy group that is gravitationally lensing HELMS18, a submillimeter galaxy at z = 2.39. We obtained multi-object spectroscopy data using Gemini-GMOS to investigate the stellar kinematics of the central galaxies, determine its members and obtain the mass, radius and the numerical density profile of this group. Our final goal is to build a complete description of this galaxy group. In this work we present an analysis of its two central galaxies: one is an active galaxy with z = 0.59852 ± 0.00007, while the other is a passive galaxy with z = 0.6027 ± 0.0002. Furthermore, the difference between the redshifts obtained using emission and absorption lines indicates an outflow of gas with velocity v = 278.0 ± 34.3 km/s relative to the galaxy.

scholarly journals Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 220
Author(s):  
Emil Khalikov
Keyword(s):  
Gamma Rays ◽  
Large Scale ◽  
Gamma Ray ◽  
High Energy ◽  
Cascade Model ◽  
Point Sources ◽  
Spectral Energy ◽  
Observation Data ◽  
Cherenkov Telescopes ◽  
The Universe

The intrinsic spectra of some distant blazars known as “extreme TeV blazars” have shown a hint at an anomalous hardening in the TeV energy region. Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations. Alternative models suppose that some of the observable gamma-rays are produced in the intergalactic cascades. This work focuses on investigating the spectral and angular features of one of the cascade models, the Intergalactic Hadronic Cascade Model (IHCM) in the contemporary astrophysical models of Extragalactic Magnetic Field (EGMF). For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance. Contemporary Hackstein models are considered in this paper and compared to the model of Dolag. The models assumed are based on simulations of the local part of large-scale structure of the Universe and differ in the assumptions for the seed field. This work provides spectral energy distributions (SEDs) and angular extensions of two extreme TeV blazars, 1ES 0229+200 and 1ES 0414+009. It is demonstrated that observable SEDs inside a typical point spread function of imaging atmospheric Cherenkov telescopes (IACTs) for IHCM would exhibit a characteristic high-energy attenuation compared to the ones obtained in hadronic models that do not consider EGMF, which makes it possible to distinguish among these models. At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM). The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones. These spectra could later be compared with future observation data of such instruments as Cherenkov Telescope Array (CTA) and LHAASO.

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