# The dipole repeller

## Abstract

Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity1 of VCMB = 631 ± 20 km s−1 and participates in a bulk flow that extends out to distances of ~20,000 km s−1 or more24. There has been an implicit assumption that overabundances of galaxies induce the Local Group motion57. Yet underdense regions push as much as overdensities attract8, but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow9. We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s−1. This ‘dipole repeller’ is predicted to be associated with a void in the distribution of galaxies.

## Access options

from\$8.99

All prices are NET prices.

## References

1. 1

Fixsen, D. J. et al. The spectrum cosmic microwave background from the full COBE FIRAS data set. Astrophys. J. 473, 576–587 (1996).

2. 2

Nusser, A. & Davis, M. The cosmological bulk flow: Consistency with ΛCDM and z ≈ 0 constraints on σ 8 and γ . Astrophys. J. 736, 93 (2011).

3. 3

Watkins, R. & Feldman, H. A. Large-scale bulk flows from the Cosmicflows-2 catalogue. Mon. Not. R. Astron. Soc. 447, 132–139 (2015).

4. 4

Hoffman, Y., Courtois, H. M. & Tully, R. B. Cosmic bulk flow and the local motion from Cosmicflows-2. Mon. Not. R. Astron. Soc. 449, 4494–4505 (2015).

5. 5

Lilje, P. B., Yahil, A. & Jones, B. J. T. The tidal velocity field in the Local Supercluster. Astrophys. J. 307, 91–96 (1986).

6. 6

Lynden-Bell, D. et al. Spectroscopy and photometry of elliptical galaxies. V: Galaxy streaming toward the new supergalactic center. Astrophys. J. 326, 19–49 (1988).

7. 7

Dressler, A. The Great Attractor: Do galaxies trace the large-scale mass distribution? Nature 350, 391–397 (1991).

8. 8

Lahav, O., Lynden-Bell, D. & Rowan-Robinson, M. The peculiar acceleration of the Local Group as deduced from the optical and IRAS flux dipoles. Mon. Not. R. Astron. Soc. 234, 677–701 (1988).

9. 9

Kocevski, D. D. & Ebeling, H. On the origin of the Local Group peculiar velocity. Astrophys. J. 645, 1043–1053 (2006).

10. 10

Dekel, A., Bertschinger, E. & Faber, S. M. Potential, velocity, and density fields from sparse and noisy redshift–distance samples: Method. Astrophys. J. 364, 349–369 (1990).

11. 11

Zaroubi, S., Hoffman, Y. & Dekel, A. Wiener reconstruction of large-scale structure from peculiar velocities. Astrophys. J. 520, 413–425 (1999).

12. 12

Courtois, H. M., Hoffman, Y., Tully, R. B. & Gottloeber, S. Three-dimensional velocity and density reconstructions of the local Universe with Cosmicflows-1. Astrophys. J. 744, 43 (2012).

13. 13

Courtois, H. M., Pomarède, D., Tully, R. B., Hoffman, Y. & Courtois, D. Cosmography of the local Universe. Astron. J. 146, 69 (2013).

14. 14

Tully, R.B., Courtois, H., Hoffman, Y. & Pomarède, D .The Laniakea supercluster of galaxies. Nature 513, 71–73 (2014).

15. 15

Pomarède, D., Tully, R. B., Hoffman, Y. & Courtois, H. M. The Arrowhead mini-supercluster of galaxies. Astrophys. J. 812, 17 (2015).

16. 16

Tully, R. B. et al. Cosmicflows-2: The data. Astron. J. 146, 86 (2013).

17. 17

Hoffman, Y. et al. A kinematic classification of the cosmic web. Mon. Not. R. Astron. Soc. 425, 2049–2057 (2012).

18. 18

Jaffe, A. H. & Kaiser, N. Likelihood analysis of large-scale flows. Astrophys. J. 455, 26 (1995).

19. 19

Hoffman, Y., Eldar, A., Zaroubi, S. & Dekel, A. The large-scale tidal velocity field. Preprint at https://arxiv.org/abs/astro-ph/0102190 (2001).

20. 20

Feldman, H. A., Watkins, R. & Hudson, M. J. Cosmic flows on 100 h−1 Mpc scales: standardized minimum variance bulk flow, shear and octupole moments. Mon. Not. R. Astron. Soc. 407, 2328–2338 (2010).

21. 21

Scaramella, R., Baiesi-Pillastrini, G., Chincarini, G., Vettolani, G. & Zamorani, G. A marked concentration of galaxy clusters - Is this the origin of large-scale motions? Nature 338, 562–564 (1989).

22. 22

Raychaudhury, S. The distribution of galaxies in the direction of the ‘Great Attractor’. Nature 342, 251–255 (1989).

23. 23

Peebles, P. J. E. The Large-Scale Structure of the Universe (Princeton Univ. Press, 1980).

24. 24

Hoffman, Y. & Ribak, E. Constrained realizations of Gaussian fields: A simple algorithm. Astrophys. J. 380, L5–L8 (1991).

25. 25

Hoffman, Y. in Data Analysis in Cosmology Lecture Notes in Physics Vol. 665 (ed. Martínez, V. J., Saar, E., Martínez-González, E. & Pons-Bordería, M.-J. ) 565–583 (Springer, 2009).

26. 26

Zaroubi, S., Hoffman, Y., Fisher, K. B. & Lahav, O. Wiener reconstruction of the large-scale structure. Astrophys. J. 449, 446–459 (1995).

## Acknowledgements

We thank J. Sorce and S. Gottloeber for discussions and A. Dupuy for her help in preparing Fig. 3. We thank K. Bowles and S. Thompson for the narration in the Supplementary Video. Support has been provided by the Israel Science Foundation (1013/12), the Institut Universitaire de France, the US National Science Foundation, Space Telescope Science Institute for observations with Hubble Space Telescope, the Jet Propulsion Lab for observations with Spitzer Space Telescope and NASA for analysis of data from the Wide-field Infrared Survey Explorer.

## Author information

Authors

### Contributions

R.B.T. and H.M.C. carried out the observations and data analysis; D.P. contributed graphics and visualization; Y.H. carried out the numerical and theoretical analysis. All co-authors contributed to the writing of the paper, led by Y.H.

### Corresponding author

Correspondence to Yehuda Hoffman.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Figures 1–3, Supplementary Table 1. (PDF 1220 kb)

### Supplementary Video

The dipole repeller. (MP4 83651 kb)

## Rights and permissions

Reprints and Permissions

Hoffman, Y., Pomarède, D., Tully, R. et al. The dipole repeller. Nat Astron 1, 0036 (2017). https://doi.org/10.1038/s41550-016-0036

• Accepted:

• Published:

• ### Cosmicflows-3: Two Distance–Velocity Calculators

• Ehsan Kourkchi
• , Hélène M. Courtois
• , Romain Graziani
• , Yehuda Hoffman
• , Daniel Pomarède
• , Edward J. Shaya
•  & R. Brent Tully

The Astronomical Journal (2020)

• ### The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints

• D. W. Pesce
• , J. A. Braatz
• , M. J. Reid
• , A. G. Riess
• , D. Scolnic
• , J. J. Condon
• , F. Gao
• , C. Henkel
• , C. M. V. Impellizzeri
• , C. Y. Kuo
•  & K. Y. Lo

The Astrophysical Journal (2020)

• ### A redshift database towards the Shapley supercluster region

• Hernán Quintana
• , Dominique Proust
• , Rolando Dünner
• , Eleazar R. Carrasco
•  & Andreas Reisenegger

Astronomy & Astrophysics (2020)

• ### Cosmicflows-3: The South Pole Wall

• Daniel Pomarède
• , R. Brent Tully
• , Romain Graziani
• , Hélène M. Courtois
• , Yehuda Hoffman
•  & Jérémy Lezmy

The Astrophysical Journal (2020)

• ### Fermi Degenerate Antineutrino Star Model of Dark Energy

• Tom F. Neiser